Why is this considered wasting semen? Can’t it be treated like ejaculation before the proper time during intercourse? In such a case it certainly is not considered in vain, because it is part of the marital act. So why shouldn’t this also count as marital relations? Or to ask differently: what is the difference between this and anal intercourse, which Tosafot permit when it is not done regularly?

Before the proper time is an accident during normal intercourse. Anal intercourse is still intercourse from a halakhic standpoint, and ejaculation during intercourse is not considered in vain. It is forbidden to emit semen not in the course of intercourse.

How is this different from intercourse by way of the limbs, which is permitted even when semen is emitted? And see an entire book devoted to these areas that sparked many discussions; see pages 74–76. Half the rabbis there are anonymous… but the main authority on the topic seems to be Rabbi Henkin; see there and in his responsa. http://img2.tapuz.co.il/CommunaFiles/52191811.pdf

And see the quotation brought by my learned colleague Yishai Kiel in “The Moral-Religious Teaching of the German Pietists”: “Alongside the ascetic spirit blowing through the writings of the German Pietists, there emerges, as noted, another voice that affirms the realm of eroticism in the relationship between a man and his wife and recoils from various ascetic practices. Thus, for example, Sefer Hasidim gives the following guidance: ‘Although they said that a snake mates face-to-face, and so do fish and man, because the Divine Presence spoke with them, nevertheless Rav said: what difference is there between this and fish? And similarly: if one wants to eat cooked meat, he may; if he wants to eat roast, he may—everything is at a person’s discretion. Only on the night of immersion it cannot be otherwise than with him above and her below, because a woman has no pleasure unless she is below… But on other nights he should do according to his pleasure so that he not think about other women, provided that it is the woman’s will, for Scripture says: “Also, without his wife’s consent, when he has relations with her, the soul…” only the child will not be good. All these things depend on the wife’s wishes and his wishes. If a man finds a good wife who agrees to his desires in these matters, then he will obtain favor from the Lord; God has desired your deeds.’ Elsewhere the author of Sefer Hasidim states that a person ‘should refrain from all pleasures that come through a woman, except for his wife.’
The words of the author of Sefer HaRokeach in this context are even more emphatic: ‘A person should refrain from everything from which the body derives pleasure through women—from looking at her, touching her, sitting near them, looking at their fine garments, hearing the sound of their speech in melody, and speaking with them, whether a married woman or an unmarried one—except for his wife; he should rejoice in her when she is pure with all the desire of his heart… Therefore he should hurry and hasten not to think lustful thoughts about women, but rather rejoice in the wife of his bosom and delight in her lovingly whenever he wishes, for she guards him from sin. Therefore, according to all the desire of his heart, let him love and honor her; then he will find the desire of his heart, and she grants him merit in this world and the next.’

To Gil: the Rabbi distinguished between intercourse, where emission of semen is permitted, and a case where there is no intercourse, where in that situation emission of semen is forbidden.
To the Rabbi: the question is, what is the basis for making that distinction? Why assume that when there is a halakhic status of intercourse, emission of semen is permitted, but in other things done with one’s wife emission of semen is forbidden? I am more inclined to think that with one’s wife it is always permitted to emit semen, as long as it is not regular (each according to his own definition), of course provided it is part of some kind of marital intimacy and not just in the middle of lunch. The main prohibition on wasting semen, it seems to me, was said about emission not in the course of sexual relations.
And that is because I find only one rationale for the prohibition of wasting semen—I am not referring to the Zohar; whoever wants to observe the Zohar and become a mystic, good for him; I am speaking about the normal rule—and that is: sexual relations not with one’s wife are forbidden.
So I would be glad to hear the Rabbi’s reasoning for distinguishing specifically between ejaculation during an act of intercourse, which is permitted, and ordinary sexual relations between husband and wife that do not involve intercourse itself.

Intercourse is always bound up with emission of semen, and therefore whenever a man has intercourse with his wife, which is his conjugal obligation, the emission of semen is permitted. But emission of semen without intercourse is not an accident, because here the emission is not necessitated by an act of intercourse; rather the semen simply comes out. On what grounds would one permit it?!
The question of why intercourse by way of the limbs counts as intercourse, and what exactly that phrase means, are not simple questions. But assuming that it is intercourse—in my opinion that is the Jewish law.
I did not understand the distinction between regular and not regular. Either it is permitted or it is not.

“The conclusion of the Talmud in Nedarim according to Rabbi Yohanan is that whatever a person wants to do with his wife, he may do, and consequently ‘turning over the table,’ which according to most medieval authorities means anal intercourse, is permitted. So too we learned in Sanhedrin 58b: ‘Rabbi Elazar said in the name of Rabbi Hanina: A Noahide who has relations with his wife not in the usual manner is liable, as it says (Genesis 2:24): “and cleave” — and not in an unusual manner. Rava said: Is there anything for which a Jew is not liable but a non-Jew is liable?!’ Hence anal intercourse is permitted for a Jew.

Tosafot on Sanhedrin 58b, s.v. ‘Is there anything,’ answered: ‘One may say that it is only forbidden when he does so regularly, so that she will not become pregnant and her beauty will not be diminished. But if most of his intercourse is in the usual manner, and only occasionally he desires her in an unusual manner—it is permitted.’ And so wrote most of the medieval authorities (Rishonim), that anal intercourse for his pleasure is permitted occasionally; so too Tosafot Rid (Yevamot 12b, s.v. tani); Riaz (Kuntres HaReayot to Sanhedrin 58a); Tosafot Yeshanim (Yevamot 34b); Rosh (Yevamot 3:9, according to the Bach’s text, sec. 6, and Beit Yosef EH 25; and explicitly in his comments to Nedarim 20b); Rabbenu Yeruham (Toledot OH 23:1); Mordechai Shevuot (laws of niddah, 732); Hagahot Maimoniyot (Forbidden Relations 21, note 4); Aguddah (Sanhedrin 7:52; and in Yevamot 3:33 he brought both answers); Ritva in his main answer (cited in Shitah Mekubetzet Nedarim 20b); Rabbenu Yonah (Sanhedrin 58b; and that is also how Birkei Yosef OH 240:5 understood him).” — quotation from the extended notes to Peninei Halakhah.
So the prohibition on anal intercourse when done regularly is because one is not fulfilling the commandment to be fruitful and multiply, or does not wish to; but when it is not regular, he does fulfill be fruitful and multiply, or at least tries to. That addresses the distinction between regular and not regular.
The prohibition for a bachelor to do this without sexual relations is because he is, as it were, forming a connection with another woman who caused him to fantasize and because of whom he ejaculates—or even to do so just like that—but not with his wife.
In short, there is reason to permit it, because the whole prohibition was said only when it is not with one’s wife, so follow the rationale.
That is: there is a general prohibition on emitting semen, as explained in Niddah 13a. The main problem with it is that it does not occur within sexual relations. I understand that you define the main problem as not occurring within intercourse, but neither of us has a source to define exactly where the prohibition lies—outside intercourse, or outside ordinary sexual relations. But my definition seems more reasonable to me, because I do not see the reason to prohibit it as you suggest. What is the problem? They are having marital relations; every couple needs different forms of intimacy, and it is stated explicitly that whatever one wants to do with his wife he may do. I am aware that some medieval authorities interpreted all this as applying only when he does not emit semen, but I brought a whole group of medieval authorities who did not explain it that way and are ruled like them. So too, as far as I know, in practical Jewish law if a couple asks whether anal intercourse from behind is permitted, they would permit it.
And there is a prohibition on living without fulfilling the commandment to be fruitful and multiply.
When he emits semen with his wife he is not violating the first prohibition, and when he does not do so systematically and does have children, he is not neglecting be fruitful and multiply; therefore, seemingly, it should be permitted.

The consideration that her beauty not be diminished is not relevant here. He can have relations with her without impregnating her. This has nothing whatsoever to do with emission of semen without intercourse.
The prohibition for a bachelor, plainly speaking, is not because of fantasizing about another woman; it is an intrinsic prohibition. Some even saw it as an adjunct of murder. Whatever they permitted was only by way of intercourse, because it is part of intercourse. But to simply emit semen without intercourse—I see no logic in permitting it.
As an aside, I also do not understand what pleasure the woman gets from this. This is a pleasure for the man that comes without intercourse. Perhaps the woman does not want intercourse. But I do not see how she wants the husband’s semen to come out for her own pleasure.
As for infrequent practice, that was said about anal intercourse. The criterion there is preserving the commandment to be fruitful and multiply and “He created it to be inhabited.” It has no connection to frequency versus infrequency. As long as he has normal intercourse around the time of ovulation, he can do everything else at other times, even frequently, so long as these are permitted acts.

A. In the time of the Talmud, without drinking a cup of infertility potion or taking a drug, he could not have relations with her without impregnating her.
B. As for her pleasure—she is happy to do his will. That does not contradict her having pleasure.
C. Some explained it based on frequency and not in relation to the time of ovulation. A practical difference, more so in the past, would be if, for example, her husband were a sailor and his frequency for the conjugal obligation was once every six months.

It is a bit hard to understand the point. If he emits semen not inside the woman, is that considered wasting semen? Obviously not?!
Or if he wants, and with the woman’s consent, to emit semen on her body but not in her sexual organ—in other places—is that permitted?

This was already answered above.

If someone has no semen, is he allowed to masturbate for pleasure?

I didn’t understand. No semen comes out at all? Then there is no problem of wasting semen, but there are forbidden sexual thoughts.

Treating wasting semen as an adjunct of murder is not Jewish law but aggadic literature. No one dies. True, the Rogatchover wanted to infer from that that even a gentile is forbidden to waste semen because of murder, but his words are a wondrous homiletic interpretation.

Plainly speaking, there is no difference between anal intercourse and intercourse by way of the limbs—that is, simple rubbing of the organ against his wife’s body. After all, those who forbid emission of semen in anal intercourse do not take into account that it has the “status” of intercourse.
The prohibition against emitting semen in these two forms of intercourse (anal and by way of the limbs) is not Torah-level, but rather a rabbinic decree because of the act of Er and Onan. According to those who permit it, anything that does not resemble the act of Er and Onan—meaning only when it is occasional, whatever exactly “occasional” means—is not forbidden.

The Rabbi here forbade “by way of the limbs”; as a rabbi issuing a halakhic ruling, that is his right. I did not understand those challenging him.

There is a major dispute among halakhic decisors whether “by way of the limbs” is permitted, and the Rabbi ruled stringently.
All the best.

And straightforward practical Jewish law is indeed to forbid “by way of the limbs,”
and only in special cases such as medical problems and the like would they permit it after asking a rabbi, and even then only in certain ways and with guidance as to what is more lenient and more stringent.

She does have pleasure:
that despite all his many and intense desires, her husband is focused on her, and everything with which he pleasures himself is around her and in connection with her.
Maybe that is a very great emotional satisfaction for a woman?
And specifically the ejaculation, which is the sign of his satisfaction—so long as it is with her and in front of her—perhaps gives her a kind of contentment?

It leads to stumbling.

Even someone from whom no semen comes out at all is still forbidden. They learn it from Joseph the righteous: “his arms were made firm,” understood like “they were scattered,” meaning that the semen came out between the fingers of his hands. According to the Ari, even there it was spiritual and that is how it worked in practice, so even if nothing physical comes out, something spiritual still does. Besides that, regarding a married man whose semen comes out before, after, or during, and the drops that continue afterward, ideally all of them should be in the place of intercourse; and for whatever did not go in, for whatever reason, one must do a tikkun yesod according to Rabbi Haim HaKohen HaHalban.

What you wrote has no connection at all to Jewish law.
Er and Onan were killed by God because they inseminated outside by means of their wives.
Nothing could be more explicit than that in forbidding this manner as well, even if the wife is the one causing it,
as long as it is not in the woman’s body.

*** Deleted (M.A.). No one asked you for the graphic descriptions of your relations with your wife. ***

According to Jewish law, can a woman stimulate her husband by hand without his wasting semen, and afterward he has intercourse and finishes inside her—is that permitted?

This thread is starting to look like a pornographic book. Yes. And enough with that.