Honestly, I prefer the MC to have the sex scenes with girls with whom he has a romantic relationship, and if at the end of the game he is only going to stay with his girlfriend, I don't like to start a relationship with more. It's how I like to play, and if I lose 90% of the scenes that's fine with me.
On the other hand, the part in which the girlfriend is willing to have an open relationship... well, in harem games I usually like it, but in those that don't have that kind of ending, no. Although possibly that is something that most of the players have liked. I'm very strange, really, but each one is as he is and likes what he likes.