You have the conversation with the player as the second thing and don't establish your table rules until after that? That is why you fail. [insert Yoda head emoji here.]If I had a player that did that I'd probably have a nasty enemy appear unexpectedly or some other serious event occur and then I'd have a serious talk with the player in question. I'd have to find out what the player thought his actions would accomplish and/or add to the narrative. I'd then explain that sexual violence will not be tolerated at my table and then I'd decide if the player needed to be ejected from the group, depending on what he told me.
I have a zero tolerance policy on this sort of thing, and depending on what the player told me, may need to reasses my friendship or acquaintance with. Physical violence in the real world cannot result from actions taken in a fictional world but that doesn't mean I wouldn't consider telling said player to pack up his **** and get out.
Wow, you joking? Wouldn't think I'd need to have a rule about sexual assault ahead of time and would be just as surprised about a player doing so as the topic starter was. I wouldn't interrupt the game and ruin the night for everyone else because of one reprehensible human being. I'd redirect the scene by introducing new elements and address the situation when appropriate. I certainly wouldn't let him continue along that path.