To bring this to a conclusion then: I think in Astarion's case, it is intentional that the relationship is not defined until the very last cinematic of the romance arc. Even after confirming the relationship (as a gameplay term) in act 2, he refuses to give it a name. "What in the world could you be?" He only finds the answer to that question after he has dealt with Cazador, when the PC becomes either his partner or his spawn. (Edit: Looking back to how this portion of the conversation started, I would assume the jealousy conversation wouldn't satisfy your needs either because as far as I remember it also doesn't offer a label, it just shows that feelings are involved on Astarion's side. )
And that is basically why in Astarion's case, while the story starts out very ... focused on physical pleasure, it is not an overly sexual story. I'd go out on a limp and say it's the opposite, especially if he stays a spawn. It's just weird, especially considering his story, that he gets picked for the sexy promotions so often. Most of it feels very "wrong" in a way.
