An initial caution - "I've never been in a relationship with a man so let's take it slow okay?" - is fine, but an outright "I'm not attracted to men" [time passes] "okay now I'm attracted to men" is...not great.
Quoting this to second it; I generally like the shape of what Ragnarok has to say here - that everyone should be ultimately available in a player-sexual way, but that that should not abolish them having individuality and personality, which includes various leanings and preferences - those leanings and preferences, because of the ultimate player-sexual end point, should not be absolutes, and if you're running against how they might normally lean, then some acknowledgement of this - while still moving forward - would be nice, and would make the interpersonal setting feel that much more realistic.
Currently, in another game I'm involved in, there is an NPC who has only ever been with other women, and generally considers that to be her preference, however, since she's been travelling with the party: while she's been physically attracted to a couple of the female party members to various degrees, she couldn't really make friends with one of them, and the other doesn't see her 'that way'... however, one of the male party members has bonded with her very closely, and they share a great many similarities in beliefs, outlooks and desires for the world around them... over the past few
months of game time, they've developed a very tentative affection for one another, and she's been working out a lot of very confused and uncertain feelings. Just recently there was a very sweet evening conversation between them where she explained that she preferred girls, and always had, but that she liked him, and that he was sweet, and kind and that she'd never been attracted to boys, and still wasn't, but that he was nice, and she thought maybe, it would be nice to try to see if this goes any further... but that she's nervous about it, and wants to move carefully... she was so adorably embarrassed about it all and he was really sweet and supportive it was a very nice little conversation. They haven't gone any further than hugs and occasional kisses so far, and likely won't for a while, as they're carefully seeing what they're each okay with... but it's really nice; this is, to me, the case of how an adaptable player-sexual npc should be done. She's clearly got, and still has, her own lesbian preference, and the ways that that impacts related parts of her character still do, but her
relationship with this player is a stronger element than her normal physical preferences, for this specific, particular situation, and so it's progressing tentatively and carefully, but it is progressing.
