People seem to treat Astarion as a non changing character that always act the same throughout the game. He changes alot trough the acts, and him reacting different to the same scenario is clearly shown in the drowtwins scene.
You dont have to pressure Astarion to have sex with the drowtwins. If you ask after killing cazador he says yes. If you ask before he says no. Showing agency from his part. Being free from cazadors gives him opportunity to heal and try to experience sex again on his own terms.