Maimonides (in a responsum, cited in Kesef Mishneh, Forbidden Relations) describes how in Egypt he would perform marriages for a “fatal woman,” even though according to Jewish law this is forbidden, since the marriage takes effect after the fact (and that too is his own innovation).

How is this different? Because that case is rabbinic?

I didn’t understand. And if it isn’t different, then what? Conducting kiddushin that involves a prohibition is assisting a transgression.

All I did was point out that there is a halakhic precedent for doing so.

A quote from Maimonides (cited in Kesef Mishneh, Forbidden Relations ch. 22, halakhah 1):

“And although there are sages who held that she should not marry, they say to her: if you find someone who will betroth you, we will not obligate him to divorce her. And they would scheme and perform kiddushin before two witnesses, and afterward the court would write her a ketubah and bring her under the wedding canopy and recite the seven blessings… and this is what we did in Egypt…”

So what is the conclusion? From now on there is permission to conduct prohibited kiddushin? At most one can remain with the matter unresolved regarding Maimonides.
But even that is not correct (though I haven’t checked it inside just now). He claims that there are those who hold this is forbidden, but they too agree that if there is a rabbi who holds it is not, he may perform the kiddushin, and they will not object or compel them to divorce. Maimonides himself testifies that in Egypt that is what they did. So in my opinion, it does not say there what you claimed. But as I said, I have not looked at it inside right now.

What’s the big deal—are there really going to be too few valid kohanim for when the Temple is rebuilt?
Let them marry according to Jewish law, and if his children are disqualified from the priesthood, so what?! Is it better that Jewish law leave them to live in prohibition?

Uzi,
I’ve now checked. It’s in chapter 21 (not 22). And indeed, as I said.
By the way, even according to the others, this is not really a prohibition but a danger.

Regarding your last response, wouldn’t it be preferable that they continue living as a couple without huppah and kiddushin? Or maybe it would be preferable for them to violate the prohibition of a kohen marrying a divorced woman one time and thereby avoid many acts of illicit intercourse?

Also, regarding your first response: if, hypothetically, you did know of a technical way for them to marry, but it involved halakhic and moral prohibitions, would it be permissible to tell them? Or in such a case does one have to conceal the truth so as not to enable them to commit prohibitions (“do not place a stumbling block,” or perhaps assisting)?

As a rule, I do not make a practice of advising how to reduce prohibitions. If something is forbidden, then it is forbidden. I have an article about this in Midah Tovah 5767.
I didn’t understand the second question. The way to marry is simply to marry. It’s just that this involves violating a prohibition (but the kiddushin is valid).

If I understood the questioner correctly, all that matters to her is to be in a state where she and the man she loves are married in the eyes of the Holy One, blessed be He (that is, halakhically married), even if that involves a prohibition (her motivation is probably an emotional-psychological need combined with metaphysical concerns). It seems to me that at the moment she understands that without a rabbi officiating the kiddushin, it is technically impossible to reach that state. What I thought was that you were refraining from answering her on that point in order not to enable her to commit a prohibition. Meaning, if she does not know how to violate the prohibition, she will not be able to violate it, so why tell her—perhaps there is a prohibition here of “do not place a stumbling block” or assisting.

Even if that is not the case, I am asking in general: if someone wants to do an act that involves some halakhic prohibition, and right now he does not know how to do that act technically, and he asks me how to do it—is it permissible to tell him? Or is it better to evade the question and leave him in his ignorance in order to prevent him from violating the prohibition (with the emphasis that this is a purely halakhic prohibition with no moral aspect, and that this person has some benefit from doing that act)?

I still do not understand the question. If he wants to do something prohibited, why would I advise him how to do it? I didn’t say it is preferable to evade the question, but rather to tell him directly that you do not advise people how to violate prohibitions.

Suppose he has a great need (psychological / physical / financial) to do that act which involves a halakhic prohibition (we are not talking about someone doing it out of spite, but out of desire). On the one hand, if I withhold from him the information about how to do that act, then I am harming him by denying him that major need. On the other hand, I am enabling him to violate a prohibition that he will probably violate, whereas until now he was coerced by circumstance and could not violate it.

I still don’t understand: if it is forbidden despite the need, why would I help him with it? How is this different from any other prohibition? And if it is permitted because of the need, then obviously one may, and perhaps should, tell him.

To Oren: why are you calling it prohibited illicit intercourse? It’s no worse than a concubine.
And by the way, the prohibition is not just one time, but applies to every act of intercourse, whether they are married or not, as far as I remember.

If one of the witnesses at the original kiddushin was a man who did not observe the Sabbath—could that be considered a problem and invalidate the kiddushin, thereby permitting the divorced woman to marry a kohen?

That is possible (if he publicly desecrates the Sabbath). This needs to be clarified in a religious court.

I have the same problem too, but I got married in a civil marriage, and my ex-husband is considered a non-Jew. During all our years together I was sure he was Jewish, because I understood from him that his mother was Jewish, and only after we divorced did I find out that only his maternal grandfather is Jewish. In such a case, is it possible to find some loophole so that I could marry my current partner, who is a kohen, and we love each other very much, since civil marriage is not recognized by the rabbinate? And I too was always sure that my ex-husband was Jewish, and only recently it became clear to me that he is a non-Jew. Is there anything that can be done? It is very important to me and to my partner to marry through the rabbinate.

Unfortunately, according to Jewish law, a woman who had relations with a non-Jew is prohibited to a kohen, even if the relations were under coercion or by mistake. Perhaps there are technical devices I do not know. In any case, if you want a solution within the framework of the rabbinate, you need to turn to them. Whatever I say will not be recognized by them in any event.