Instead of giving GM4Him a hard time for being Christian, maybe engage with the topic in good faith...
He doesn't want content in the game to be locked away from him with the explicit depiction of sex, his reason for not wanting to see it are his own, but hardly unheard of. I suspect he doesn't have a problem with the depiction of sex in art. It sounds more like a franchise that was important to him as a younger fellow is now being turned into something else, something he can't engage with the same way he did then.
Unless you want to continue to argue about whether or not the depiction of sex has any place in a video game; tell him he's being unreasonable, that there should be a way to sanitize the rating for people who want it, or agree with him that it has no place in the Baldur's Gate franchise.
I don't agree with him on this, I don't think the sex is separate from the story, and I don't think it or nudity is out of place in the the setting. Baldur's Gate is going to be a story with people having sex, so far none of it is compulsory, and who knows if refusing to participate will even lock you out of content.
Just out of curiosity GM4Him, do you refrain from watching movies that have sex scenes in them? Or is it just the supposed shift within a franchise that you find off-putting. As for Star Wars, the sequels are floundering because they couldn't tell a coherent story with them, when you tell a good story you can do anything you want.
