Yeah, I don't think it really matters. At least, from my experience of playing this.
For example, in my game I have it set to medium (default setting). New game, girlfriend doesn't really have any points with anyone. Lust levels are low still, so no one is horny at the moment. I see the girlfriend and Montana in Montana's room. I go there, and they are having sex on the bed. No relationship points at all for Montana, Montana and the girlfriend's lust is basically zero, but there they are. Banging away.
So yeah. It seems nothing really matters. It is all just random.