This is a Torah-level prohibition of “and guard yourself from every evil thing.”
As Tosafot wrote (Avodah Zarah 20b, s.v. “shelo”), this is a full-fledged exposition, not merely an asmachta.

Maimonides and Sefer HaChinukh do not count “and guard yourself from every evil thing” among the commandments. It seems they see this as good advice and not as Jewish law. How can one command something that sometimes happens to a person against his will, like a seminal emission in a dream?

And this is what Maimonides writes (Laws of Forbidden Relations, chapter 21, laws 18–21):

‘It is forbidden to emit semen in vain. Therefore a man should not thresh inside and winnow outside, and he should not marry a minor who is not fit to bear children. But those who commit adultery with the hand and emit semen— not only is this a grave prohibition, but one who does this sits under excommunication, and regarding them it is said, “your hands are full of blood,” and it is as though he has killed a soul.

And likewise it is forbidden for a man to look at women while they are standing over the laundry. Even looking at the colored garments of a woman he knows is forbidden, lest he come to sexual thoughts.

And likewise it is forbidden for a man to intentionally arouse himself or bring himself to sexual thoughts; rather, if such a thought comes to him, he should divert himself from idle matters to words of Torah, which is “a loving hind and a graceful doe.” Therefore it is forbidden for a man to sleep on his back with his face upward, unless he tilts slightly, lest he come to arousal.

And he should not look at animals or birds while the male is mating with the female. But those who breed animals are permitted to insert like a brush into a tube, because they are occupied with their work and will not come to sexual thoughts.’

So if, at the time of doing laundry, when only their arms are uncovered, it is forbidden according to Maimonides, then all the more so in a mixed swimming pool.

Best regards, may his light shine forth and spread outward.

This is, of course, a stupid halakha, and it should simply be erased from the books and we should move on. Good luck to the naive fools who take these laws seriously.

Y.D.,
“How can one command something that sometimes happens to a person against his will, like a seminal emission in a dream?”
There is no command about that. The command is not to bring oneself to sexual thoughts. Meaning, for example, not to go work at a mixed swimming pool. That’s all.

Yerachmiel, I always enjoy and am impressed by forceful statements. But what can I do—I disagree, and certainly not necessarily. Go take a look at the passage of “when it is unavoidable and he does not intend it.” That’s all.

And of course also the Ritva at the end of tractate Kiddushin.

Michi, you have excessive piety in you, and great advocacy on behalf of the Holy One’s children, that you compare our case to “unavoidable and not intended.” If only for all the Jewish people that working at a mixed swimming pool would be connected in their minds to that passage.

And regarding the Ritva at the end of Kiddushin, yes indeed. But the questioner did not specify whether he has already reached the level of the Patriarchs, over whom the evil inclination had no control, and therefore the one answering him must take into account the possibility that he has indeed not reached that point.

Of course, the Ritva too did not write his words for ministering angels.
And regarding “unavoidable and not intended,” this is a common mistake. In the medieval authorities (Rishonim) on chapter 2 of tractate Pesachim it is explained that “unavoidable” also applies when there are two paths before him, and one is only a bit longer than the other. Taking the longer path is still considered “unavoidable.” Look carefully.

I’ll just add that what confuses people is the assumption that “unavoidable” means coercion. But that is not so, since in that passage the discussion is about lack of intent, not coercion. And the explanation is as simple as an egg: the moment he takes the shorter route and there is a prohibition there, it is clear that he did not go for the sake of the prohibition but because it is the shorter route; therefore he is considered as not intending the prohibition. By contrast, if the two routes are the same length, his taking the route with the prohibition proves that he did so intentionally with respect to the prohibition. And so too regarding the pool. He is doing it for his livelihood, and therefore he is considered as not intending the prohibition (even if there is one).

The source of the prohibition is in the Mishnah: “Do not increase conversation with the woman.”

You explained “unavoidable,” but what about “not intending”? And what is “unavoidable but intending”?

As I understand it, “intending” means that at the time of the act he intends to derive benefit from the prohibition, even if he got there in a state of “unavoidable,” and that is forbidden according to everyone.

Therefore one should forbid working at a mixed swimming pool.

In the plain sense of the passage, the intention does not refer to the time of the act but to the decision to go in that direction. He does not intend to go in order to commit a prohibition, but for some other reason.

The “possible/unavoidable” refers to going there. The intention refers to the time of the act.