>>
Before actually attempting it in an DM session, you should probably discuss it with the players beforehand. Like hey, sexual themes / death / etc could be possible, including that it could be forced on your character without their knowledge or consent.
If agreed upon, in most cases the victim should still be allowed at least one or several chance(s) to either resist the attacker, or call for some form of help (Whether that be from other PCs or NPCs). It is dependent on the situation at hand, obviously - but don't just drop it on a random player without at least giving them a heads up that it could happen during the course of a game.