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.
I do not think it is the same person at all. Astarion said that he couldn't make himself bring that darling boy to Casador, so he ran. And then was caught and locked up for it.
