Actually, that’s exactly why I suggested that the goal is not that the heart turn away, but that the heart turn away through a woman. Otherwise the command in the verse is really difficult, because even if my heart turned away unrelated to women, apparently I haven’t done anything by refraining from taking many wives—so why would we be punished or warned about that? And why warn only about one of the ways my heart might turn away and not all of them?

Now regarding what you asked, it could be explained like this: taking many wives is a condition for violating the prohibition (in other words, it is an exemption for someone who has few wives), but it is not the content of the prohibition, which is, as stated, “that my heart turn away through a woman.” So basically, in order to violate the prohibition my heart must turn away through a woman; however, if I had one wife, then I am exempt, because the Torah of course permitted a man one wife, and therefore having few wives constitutes an exemption. Likewise, if I had many wicked wives and my heart still did not turn away, I also did not violate the prohibition, because I only lack the exemption, but I did not violate the content of the prohibition. And indeed, if I had many righteous wives and nevertheless my heart turned away because of one of them, then since on the one hand I violated the content of the prohibition and on the other hand I have no exemption, it follows that I would be held liable.

I don’t see the logic. Why should the heart’s turning away be specifically through women? If there is a problem with the heart turning away, then that applies in any way. You won’t be able to escape a certain degree of formalism. In the article mentioned above I suggested an explanation of Maimonides’ view of the first tanna that seems to me the most reasonable: that taking many wives, even righteous ones, turns the heart away. According to this, the heart turning away in some other way is the reason for the verse, which of course should also be avoided, but it does not involve a formal prohibition.

I’m not sure that this is different from your suggestion.

Hear me out, it seems to me that it really is specifically through women, because it’s strange to me that the Torah would insert such a simple and general reason into a commandment—kind of like what you said about how there can’t be a law in the law book telling you to obey the law. So the general reason, that his heart should not turn away, does not sound like it belongs as part of the reason for the verse of the formal definition of a specific commandment. I will say, though, that I just thought of what is actually a pretty nice explanation here, in my humble opinion, for why specifically through a woman. I’m reading your article about the difference between the sciatic nerve and the prohibition of leavened food, and there you bring up that some commandments are based on historical remembrance (and perhaps circumcision is similar as well). I think one could say something similar here: that the Torah wants to establish as a prohibition that my heart should turn away through a woman. Why? “And to Adam He said: Because you listened to the voice of your wife and ate from the tree of which I commanded you, saying: You shall not eat from it.”
Although the Torah could not forbid marrying a woman, and it also could not forbid some amorphous prohibition like “your heart shall not turn away through a woman,” it therefore prohibited taking many wives (which is one particular way of mitigating the turning of the heart through a woman—either that works or it doesn’t), and it told me when I violate it—when one of those wives turns my heart away. And all of this is really only a remembrance of what is told in the Garden of Eden story. And you should not say that the Torah is trying to give you good practical advice for how not to have your heart turn away—because, as you teach in many places, the commandments in the Torah are not moral/utilitarian in a practical sense, but religious in character, and this explanation sounds a bit more “religious” to me (whatever exactly that means).