Tavlin upon learning that Astarion was a vampire was, of course, horrified. he’s undead, he’s wrong, he’s wicked, and by decree of his goddess he should ram a stake through his heart. but he’s gotten to know Astarion, and he’s not really all those things. well sometimes he his, but Tavlin likes him well enough. Hard not to bond with the people sharing the same living nightmare as you. and also Tavlin hesitantly allows Astarion to drink from him, after all he is a cleric — a healer! — isn’t it his duty to help those in pain? after, he finds it incredibly thrilling, though he represses that feeling as best he can. In truth it’s a strange work around to intimacy. Though Astarion is exceptionally close, his lips against his neck, his body pushed against him, technically it is nothing more than feeding. He becomes a very willing subject to subdue Astarion’s thirst, though perhaps not the best choice; an underfed, iron-deficient elf is not a very amazing source of nutrition.

Astarion and Halsin wouldn’t consider themselves partners for a few years. I think Astarion would make more effort to speak with Halsin after Tavlin and Halsins relationship begins. Astarion perception of Halsin at that point was that he was a big horny dope, and hadn’t bothered too much to pay attention to him. Astarion was preoccupied with figuring out his feelings and fears with Tavlin as well as surviving the next day. After Cazador’s death, everything becomes more manageable, and he finds himself genuinely interested in learning about Halsin. I think they’d be good for each other. Manage to actually talk through their repressed traumas, when they have opposing feelings when thinking back on them (Halsin painting them in a brighter light than they are, Astarion’s disgusting and hate of it all). When the netherbrain is defeated, Astarion stays behind in Baldur’s Gate with Tavlin who rebuilds and revives Selûne’s church, and Halsin travels to moonrise to rebuild there. They visit each other frequently, and here I think is when Astarion and Halsin would actually fall for each other.
Astarion and Tavlin (and Halsin) are allowed to freely express themselves sexually and emotionally with other partners, though they find that the emotional aspect of a relationship is very difficult after experiencing such a unrelatable trauma. Tavlin mostly takes advantage of the openness of their relationship, since he was so inexperienced at the beginning of it. Astarion wants to, but it brings up more trauma than he would like — not being close with his sexual partner, putting trust into someone he knows barely anything about. Halsin on the other hand, is the perfect option for exploring his sexuality — and he is very enthusiastic to do so. They go from friends, to friends with benefits (i suppose lol), to finding themselves incredibly invested in the other. I think it takes a few visits for Astarion to realise that he’s in love with Halsin, because he’s only really felt it once before and it was so difficult the first time to admit that he was in love with Tavlin. I think Halsin felt very strongly for Astarion very quickly, but he was happy to be whatever Astarion needed him to be, not pursuing his feelings. They both talk extensively about it to Tavlin, and he would have a field day with it. Trying to get them alone together, stupid cliche romantic things that he had read about in his romance novels. I think at some point it would just click. They’d stop being awkward and apprehensive, and Astarion would tell Halsin that he wanted it be more.

yes indeed! Tavlin is half-fey and his fey ancestors are sexually fluid. They do not reproduce asexually, rather then can convert their sexual origins in order to reproduce through impregnation or becoming pregnant. Tavlin was born intersex because of this, having both partial testies and uterus, and his outward genitalia being a penis. He is taken from his fey family and raised in a rigid religious environment in Baldur’s Gate where gender norms are forced upon him due to his outward sex. He is unaware of his ancestory and fey abilities until during the game canon. And post game he learns how to manipulate his body the way is fey ancestors are able to. Tavlin is a gender non-conforming character who would have been originally raised in a society that has no set gender norms but has them forced onto him.