Her decision in Act 3 seems to be far more random.
I wonder if it depends on something after all. With me, she always chooses the option to "let go" of her parents and go the way of Selune.
Someone mentioned that it depends on whether you get her memory about the smells of the city when wandering through Baldur's Gate. I am not sure if that's it though.
As for the "pushing buttons til sex" I doubt anyone does that - unless you are doing that weird sex speed run thing. In general I'd say people pick the character they like and want a deeper story connection with. Could it be better, especially in Act 3, most definitely, but that is a general problem especially of the late game, where all the character interactions and banter seem to dry out. Some more interactions with your friends/romantic partner during the late game would be nice and I have seen tons of good suggestions for those.
The marketing is overall way more thirsty than the game itself. (Poor Astarion.)
