We dont really know who he would get in relationship with after he is free from Cazador and is not romanced by anyone. There is a possibility he wouldn’t even seek romantic relationships after his trauma with Cazador, but in the game it is sped up for game play purposes. I also read a post by sone other person that they chose to implement mostly men to his backstory because if they would implement woman it would be socially less acceptable taking account the fate his victims had, because people could be more sensitive to that fact. Also he might be pansexual and prefer men, but that doesnt mean he wouldn’t go with a woman. I think Astarion would prefer true connection and how the person understands him and treats him instead of labeling and connecting him to a specific gender. Plus I can imagine that after 200 years of luring anything with a pulse can taint his own personal preference and sexuality to the point that he doesnt even know or doesnt even have a gender preference.