My straight girl character has been approached by other straight girls before, and I've also gotten other straight girls I'm friends with to accept my character's advances. So yeah, I'm pretty sure sexuality is a little malleable and fluid in game just like it is in real life. Especially if you're close with the character in question and one or both of you is drunk!
I'm not sure it's "fluid and malleable" as it is IRL so much as mostly unimplemented. As far as I'm aware, it doesn't actually mechanically influence anything besides the genders of professors and some storyline NPCs and one specific scene during adult streaming. Maybe it affects NPC interest? If so, it's a numeric multiplier and not a gate.