From Astarion's backstory with his old master, he is definitely bisexual(not by choice imo).
Shadowheart is not a playersexual companion(she commented on me getting with Astarion, but she has no interest in a girl), and I like how there are characters who have their own preferences. It completely destroys character writing, if you write them to cater to political correctness.
Astarion flirts with both female and male party members via banter, seems to me he made his choice.
Shadowheart's party romance scene with kiss is available to both male and female PCs. She turns down every male party member's advances done via banter (Astarion and Wyll super fast, Gale takes a few exchanges) and her only female option is Lae'zel whom she hates, so that's a no-go for flirting. Last thing to check is if her "super attractive" person in the tadpole dreams switch gender based on the PC. I know it's male if you play a male character, but I don't know if it is still male if you play a female PC.
Note: there is a banter where she mentions that she's not into long term relationship. Note 2: the only thing she remembers about herself is: like black orchids, can't swim and cleric of Shar on a secret mission. Note 3: you can learn that she is supposed to get her memory back once she finish her mission and bring back the artifact to the Shar coven in Baldur's Gate. Act 2 is going to be wild with her I think...
Wyll comes off as bi-curious in a banter with Astarion.
I can confirm Shadowheart said her dream person was a male, as a female PC.
