All companions are - for the sake of the player, not the story - playersexual. It requires some suspension of disbelief, but I think it's a good choice because personally I've always considered it a little lame to lock a player out of certain romance just because of their character creation choices. Forcing someone to roleplay a bodytype they don't want to just for the sake of romance is a bit much. Just my opinion, of course. I fully understand if others disagree, as it can take away from the story to have someone fall in love with your skinny female gnome while they canonically only show interest in tall male bodybuilders (just an example of extremes).
I do admit Astarion's story certainly has the most emphasis on previous partners, though. Other than Gale with Mystra (which can be excused by her being a goddess having more sway than her being a woman), you don't really get much information regarding the other origin companions except for hints about their preferences and knowledge about their experience. For what it's worth, he does mention he was forced to seduced and bed a lot of people for Cazador, and he's over 200 years old, so the few you see are just scratching the surface. 7 spawn seduced 7000 people. You'll just have to imagine everything you can think of (within legal limits) is amongst those, I guess.
