I’m all for companions having the potential to change their opinions and alignment, for good or bad, in the course of the game, depending on what happens in the story and the relationships they build with the PC and possibly even other NPCs.
For me, those changes have got to make narrative sense and be rooted in the established character and motivations of the companion, and should in most cases be independent of whether or not the PC is romancing them. Plus one of my cRPG companion bugbears is when we have to be super-nice to build a relationship while they can be as prickly as they like, and that’s additionally unappealing in the context of a romance arc. For me it’s important that relationships, whether platonic or romantic, between characters who have very different outlooks feel as though both parties are struggling towards mutual understanding and respect, rather than my PC having to carefully pick nice dialogue choices while the NPC gets to be antagonistic.
With respect to Astarion specifically, as others have said, he’s had a terrible couple of centuries and that no doubt is affecting how he behaves now. He’s clearly selfish and motivated by obtaining power, but it feels plausible to me that depending on what happens with the PC and story, he could either go full blown evil, or mellow to a neutral that would regard acts of altruism with amused tolerance rather than outright disapproval.
And given he seems to view sex as both a diversion and a power play, it doesn’t actually make a lot of sense that he’d refuse to sleep with the PC simply because he didn’t approve of them. So I suppose it would make more sense to me if a good aligned character could attempt a romantic relationship with Astarion, but it is hard to imagine he’d have enough love or respect for them to change for their sake. Such a pairing sounds more likely to me to end in a broken heart than eternal, undead bliss
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