I think you should just let it go. It sounds entirely benign. You will have many more troubles if you try to change this old habit of his. Just think through all the difficulties that he and you will encounter, and assess whether it is worth it. To me, it does not seem to be worth it, but I am not you, plus, I know that there are specific dynamics in kink relationships that I am not aware of (being vanilla), so there might be some subtle power play or something else that is lurking in the background - you would know better. On the surface, it seems to be a minor, benign issue.

the only thing that worries me a lot is that "he swore on our daughters lives that it won't happen again." I find it horrifying that he would do that, and that you would accept it. His is not a habit that would be easy to break - you should both realize that (and you do - you wrote that old habits die hard, so you are in touch with reality). So, chances are, even if he tries, he might slip. And then what? Will your daughter die?.. He swears on his daughter's life so easily?..

Another thing that is horrifying is the lack of reverence towards human life, demonstrated by his swearing on his daughter's life when he is talking about something fairly stupid and minor - dirty talk with women is fairly stupid and minor, and the life of his daughter is neither stupid nor minor, so for him to put those things together... bizarre