Well, Samarra will have sex with you regardless (after your first agility training, which is required to complete all quests and win), without any mind control. But otherwise, since you only need four of the five, you can win without manipulating any of them into sex.
Actually, I wonder what the minimum number of sex partners to win the game is…
[/ISPOILER]).
Sex with Belle is optional, yes. She won't be as friendly with you, and you won't get the option to tell her to stay instead of her going off to the pond at the end, but I am told you can refuse her (haven't actually done this myself).
Samarra is a bit weird - you can partner with her in the epilogue if you have sex with her during the game, but you can't have a relationship with her during the game. And you get her charisma point even if you don't have sex with her at all after the first training session.
Of the people you can have a relationship with, Nell and Evelyn are old fashioned and you *can't* have sex with them unless you're in a relationship with them. If you're going to get a charisma point from Primrose, I think you have to accept the offer of a boob job after keeping her out of jail in order to get the night time invite... again, haven't actually tested that.
But Callie is the only character who you must have PIV sex with at least once *before* you get the option to become her boyfriend and get a charisma point. Because getting Callie's charisma point requires so many steps (cast the spell, have her do strip shows x3, ask to have sex, be sure you've seen enough of her fashion outfits, be sure you've worked for her enough), she's usually the one I skip having a relationship with if I'm going to skip anyone. (Technically primrose requires almost as many steps, but most of it involves reading books with her that you're going to have to read anyway, so it doesn't feel as much of a chore.)