As for the "I'l be yours until morning at least" this happens before you enter into a relationship. At this point in the story you can explore your options with every companion without consequence. Astarion sleeps with Lae'zel if you don't sleep with him the night of the tiefling party, but again this is before the player has entered into a relationship with anyone.
It's maybe also worth noting that during the act 1 encounters he is sleeping with you (or Lae'zel) not because he likes it, but because he is looking for protection. Depending on the Act 2 scene you get, he even admits that - even though he likes you - thinking about intimacy (I assume he means sex because he seems to be fine with kissing - maybe, hopefully? ) fills him with disgust and loathing. So bringing up those scenes as an argument for why it should be ok to cheat on him, feels weird. Especially if you got the Act 2 scene in which he apologises for spinning you along.
His rapture - the only good act 3 sexy time encounter in my opinion - is also not being free, but being safe.
