So, when you create your character. You can be given the option on choosing your sexuality. I think sticking with the options of simply, "Attracted to Men", "Attracted to Women", "Attracted to All", would work. Depending on which one you'd choose, certain scenes would play out differently. So if you choose "Attracted to Men" and were let's say the Marin quest line, certain lines would change and maybe you'd get a condition like Tired or Aroused, but it would be like, Uncomfortable. Just allowing all encounters open if you wanted but having certain ones changed based on your sexuality. So if you were "Attracted to Women" and you were up against a Male enemy, Any actions you do or they do, your lust stat won't go up as fast or at all.
Is there anything else you need explained or does this work?
You can get living underwear without female partners on! Specifically, if Marin is off limits to you, the shop of horrors will sell it to you directly. Right now, it's only handholding I think that's out of reach for players that don't like female partners (and that hopefully won't take too long to change, though tbf, only girl npcs have handholding scenes at the moment anyways.)