Way I see it, having sex in these games is like getting a beer with someone. You're buds, you're horny, what's there to think about? I don't think locking sex behind walls of text is what makes good characters (that isn't to say that can't work), but if the character grows closer to you over time that makes them good.
It's a weak example because it's about dominance rather than love but look at Berwyn. You have enough of the right kind of sex with him, and he acts differently with you and will even father a child with you. Now just replace the "sex to get different sex" with different activities that showcase your growing bond as two people rather than fuckbuddies. Example: getting closer to Brint by playing skeeball together, chilling by a lake, getting beers at a pub other than any one Cait would frequent, going to a sports game, and it showcases the emotional connection rather than the physical. Show the player that the two of you have bonded yknow?