It honestly looks like he feels so emasculated by Tav making his life decisions for him that he doesn't dare/feel confident enough to express his sexuality.
Yes, it's true, sometimes virtue doesn't bring happiness. It is good, but it hurts and it takes time. Ascended Astarion is a vice, a power, he gets a new life and his sexuality is a hurricane. It only happens with Tav as an ally who says "yes" to vice. Well now with more sadistic kissing when Tav is the victim-(which is irrational for story)
