I will agree with some here that it does seem like you're refusing to see the evidence of Astarion having preference for females too.
Why are you discarding everything that has been pointed out? Do you just want to be told that he's gay or?
Because that would be a flat out lie, I'm afraid.
There's plenty of nods to Astarion leaning both ways, on more than a couple accounts. As he was intended.
Just because Sebastian wasn't female or there weren't a square 50/50 male to female ratio in that one particular cell doesn't indicate that he prefers men.
Besides, that "sweet darling boy" and Sebastian is the same person he's talking about, they're not two different people.
In the end Astarion was written to be "a man of many tastes" (aka. leaning both ways) Which I think he definitely comes across as on more than one occasion. If someone begs to differ is irrelevant, really.
You're free to headcanon whatever you wish however.
