I think Astarion is demisexual so he does enjoy sex but only when there's a deep connection with the other person. If you reject his advances in the second sex scene in Act 1, he says that it's a shame because the first one was different from his other experiences and he might remember that one as something special. He also decides to have sex with Tav after defeating Cazador too so he might be overcoming his trauma at his own pace and with the right person by his side. About his evil route, I did watch how the final sex scene plays and...he basically becomes like Cazador, so nope, I'm not going down that route either.
Astarion explicitly says that he can't enjoy it because even he wants it and it's with a person he really cares for (Tav), his mind takes him back to everything that has happened to him and it taints the experience.
