Astarion is not ace, no. But with the amount of trauma he has and him admitting to using sex as a tool - one he is actually disgusted by - I think he might be too traumatized to actually know what his sexuality is at this point. There is an argument to be made for demi, but with the amount of trauma that's hard.
But, Tav can be demi or ace and still be attracted to Astarion. And given his story arc, I actually see the possibility of this being implemented.
Ideally I thought Halsin a good candidate for demi and ace representation - obviously his writer disagreed.
