Following up on this question, would the same ruling apply to a non-Orthodox wedding as well? Or did you mean that only regarding a wedding of religious people who are celebrating a life of prohibition is there something objectionable, but regarding secular people, they aren’t really in the category of deliberate transgressors anyway and aren’t aware of the prohibition, so there’s nothing objectionable here?

The distinction is not between religious people and those who aren’t. It seems reasonable to me not to go even to the wedding of secular homosexuals. A non-Orthodox wedding is different even if the couple getting married are believers. Here there is no celebration of a prohibition. At most, there is no valid marriage here, and you are making them happy as they begin living together.
Also, in a secular wedding where the couple is going to commit prohibitions, there is no problem attending. They are marrying lawfully, and they have the option of living in accordance with Jewish law. The fact that they choose not to do so is their decision, but it is not rooted in the ceremony itself.

Why is there no prohibition here of strengthening the hands of transgressors?

And what if we’re talking about atheists?

What prohibition is there in that? There is aiding, and there is "do not place a stumbling block." Where is there a prohibition of “strengthening”? (I’m not referring to specific places where some prohibited giving encouragement.) Especially since there is no real strengthening here, because he will get married with me or without me. And it is not comparable to buying in a store owned by Sabbath desecrators, where I myself am supporting them financially, and it seems to me that even there there is no real prohibition.

Even in the case mentioned in the Talmudic text, it’s talking about a situation where even without me he would commit the transgression. After all, even without my “well done,” someone working during the Sabbatical year will keep working. And also in the case of buying in a store of Sabbath desecrators, there’s no encouragement to commit a transgression in the actual act of my purchase, so דווקא in such a case there’s more reason to say it should be permitted.

Not at all. Those who transgress the Sabbatical year laws depend on public legitimacy. In purchasing from a store there is also financial assistance, but weekday shopping is not direct assistance to Sabbath desecration.
But as I said, I am not familiar with a prohibition of “strengthening.” Only specific cases that were prohibited (or not recommended) because of this. So these distinctions aren’t important for the matter at hand.

“Also, in a secular wedding where the couple is going to commit prohibitions, there is no problem attending” — the difference is that in this case the wedding itself is not prohibited. Whereas a homosexual wedding is itself prohibited: “You shall not do as they do in the land of Egypt” — and what would they do? “A man would marry a man and a woman a woman” (Torat Kohanim). And apparently the law should apply to all participants in the ceremony and those who come celebrate there, since they are transgressing “You shall not do as they do in the land of Egypt.”

With that kind of expansive interpretation, you won’t get very far.

Even if we don’t extend it to the wedding participants (and there’s no reason not to — after all, they are part of the celebration and the ceremony), still, the difference between a wedding that is itself prohibited (a same-sex wedding, or a wedding of a Jewish man and a non-Jewish woman) and a secular wedding (heterosexual) that is itself permitted and perhaps even desirable, is a major difference.

The wedding is not prohibited.

Which wedding is not prohibited — a Jewish man with a non-Jewish woman, or a man with a man?
And how is it not prohibited?
After all, the Torah explicitly says, “You shall not intermarry with them” regarding marriage with a non-Jewish woman.
And in the words of the Sages it is explicit that same-sex marriage is an act included in the prohibition of the practices of the land of Egypt: “I referred only to statutes established for them and for their fathers and their fathers’ fathers. And what would they do? A man would marry a man and a woman a woman. A man would marry a woman and her daughter, and a woman would be married to two men. Therefore it says: ‘You shall not walk in their statutes.’”

The wording of the Torah does not necessarily compel a prohibition of marriage. Regarding a priest and a divorcée, there is a dispute about the wording of taking: whether the prohibition is on intercourse or on taking in marriage. Regarding a male, in Maimonides’ wording it is clear that the prohibition is on intercourse, and at the moment I don’t recall a discussion about marriage.
It is worth comparing Maimonides’ formulations at the opening of the laws of forbidden sexual relations; there you will see explicitly that the issue is intercourse and not taking in marriage.

Are there examples in which the Torah prohibits marriage, and the marriage is not valid at all?
Because intuitively it seems to me that the marriage of two men is not valid at all, and maybe from here it follows that the Torah does not prohibit it (because there is nothing there at all).

Here you’ve raised a very broad question. First, there are such cases, for example the forbidden sexual unions. In those cases, betrothal is not valid, unlike cases involving mere prohibitions. Clearly, between homosexuals the betrothal is not valid.
But at the beginning of tractate Temurah, Abaye and Rava disagree on the question of “whatever the Merciful One said not to do — if one did it, does it take effect or not?” In other words, when someone acts in a forbidden way, is the act halakhically effective or not? There it appears that according to at least one opinion, even if the act is not valid there is still a prohibition because he violated the statement of the Merciful One (he violated God’s command).
Beyond that, specifically regarding prohibitions of marriage and intercourse, there is room to prohibit the marriage even without the betrothal being valid, since with marriage there is not necessarily a question of validity or invalidity. It is simply living together.

It’s true that the wording of the Torah does not prove it, but the wording of the Sages in Torat Kohanim (which I quoted above) certainly does point in that direction: “A man would marry a man and a woman would marry a woman… therefore it says: ‘You shall not walk in their statutes.’”
And although one could perhaps say that the Sifra uses the language of marriage not necessarily literally, and really means male homosexual intercourse (just as Maimonides ruled regarding women rubbing against one another under the law of “the practices of the land of Egypt,” and brought this Torat Kohanim), nevertheless from the words of the medieval authorities (Rishonim) regarding the prohibition of walking in gentile statutes, one can see that the prohibition includes three types of acts:
1. Ceremonies indirectly connected to idolatry and the customs of idol worshippers (even when the act itself is not idolatry).
2. Ceremonies and acts indirectly connected to sexual prohibitions.
3. Empty acts that undermine “You shall be wholehearted” — matters such as divination and the ways of the Amorites.

And a homosexual wedding meets criterion 2, since it is a ceremony of a relationship closely connected to forbidden sexual relations. Therefore it seems that the Sifra meant this quite specifically.
The reason Maimonides did not mention this prohibition is that such a wedding (as opposed to the act itself) was something unimaginable in his time (unlike in the time of the Sages, when it existed among some peoples). And is the tanna supposed to list, like a peddler, all the acts included in the prohibition of the practices of the land of Egypt?

(P.S. True, someone who wants can disagree, since the matter is not explicit in Maimonides, but I took advantage of the discussion to present what seems correct to me.)

To MY and to the Rabbi

A distinction must be made between marriage and betrothal. While betrothal is a legal effect, it is still not clear that there is a prohibition involved even according to the view that says that even if it does not take effect it is still prohibited. This is a strange dispute altogether. It is a logical issue that does not really belong to disputes among the Sages. It’s like saying that there is a formal prohibition on attempted murder (an attempt and not an actual murder). There is no such prohibition. There is a prohibition on murder. Naturally, someone who tried and failed still did something bad. And naturally, it is also forbidden even just to try for no reason (“let’s try, let’s see”), like one who shoots arrows negligently. But that does not apply in the case of someone whom it is impossible to murder, where there is no prohibition on trying to murder him (in the sense of an inevitable result). But marriage, simply speaking, is not a legal effect but a reality, like murder. Therefore, while betrothal does not take effect in forbidden unions, marriage (at least according to the language of the ancient world) does take effect — including among same-sex couples. Indeed, from the wording of the Sifrei it appears that the marriages are prohibited, but that was not ruled as law (at least in the area of sexual prohibitions) by the medieval authorities (Rishonim). They apparently understood that this is perhaps a conceptual or aggadic introduction to the sexual prohibitions that come later.

However, regarding prohibitions of walking in gentile statutes, one indeed has to think about it. Maimonides did not explain, but it does not seem to me correct to say this is because in his day it did not exist. Maimonides has no problem anywhere in the Mishneh Torah mentioning dozens of cases not relevant to his time if they are mentioned by the Sages. He could easily have counted this act because it is mentioned in the Sifrei. In any case, I do not know how many of the prohibitions of walking in gentile statutes apply today. The three areas MY mentioned are actually still included indirectly under idolatrous customs. The third clearly so, but also sexual prohibitions (the reality of cult prostitutes, and also “the Jewish people worshipped idols only in order to permit themselves sexual immorality in public,” which may itself have been tied to the cult of prostitution. It may be that sexual immorality was, among other things, itself a kind of ritual). Today homosexuality does not stem from idolatry but from modernity (progress, in foreign parlance). The proof is that the rest of the sexual prohibitions are accepted today by humanity, even beyond the commandments of the Noahides (not from piety but from wisdom. To the point that cousin marriage is not customary [for reasons of genetic defects in the fetus]). But it still requires further analysis.

In the seventh line from the bottom: “the third clearly.” By the way, regarding marriage as a reality and not a legal effect, the Bach holds in Maimonides’ view that although betrothal does not take effect with a non-Jewish woman, if a Jewish man marries a non-Jewish woman the marriage does exist (presumably with respect to the prohibition of a married woman under the seven Noahide commandments, meaning that she would become forbidden to other non-Jewish men). That is how the prohibition of “You shall not intermarry with them” works according to Maimonides, even though the betrothal does not take effect (and then one has done the act and it takes effect). This also explains why, according to Maimonides, a priest who betroths a divorcée is not flogged even though the betrothal does take effect, until he has intercourse with her. Because the prohibition is in marriage (and intercourse after betrothal is like entering the bridal canopy), and without betrothal — according to the plain meaning of Maimonides — there does not seem to be a prohibition of a divorcée to a priest in plain intercourse, beyond the general prohibition of an unmarried woman without betrothal. This also explains Maimonides’ view that the commandment of betrothal is a commandment conditioned on marriage. (At the opening of the laws: if a man wants “to marry” a woman — not merely wants intercourse — after the giving of the Torah, he must first betroth her. If he did not want to marry her and only had intercourse, then it seems he violated the prohibition of a promiscuous woman, but not that he nullified a positive commandment, although from the count of commandments it appears he also violated a positive commandment.) If marriage were a legal effect, it would be impossible to nullify this positive commandment according to his view.

That doesn’t seem right to me, especially since Maimonides also brings laws that are not practical.