IIRC, the sex scenes are influenced by you and the NPCs' inclinations, relationship, and the dominance/submissiveness levels (if they have one available).
Some NPCs might not feel comfortable about your penetrating them or the other way around unless you become closer to them, shown from the green through red on asking. You could also always force them, if you have certain inclinations (specifically the dominant ones) though your partner might not like it when shown afterwards.
Your partner becomes less restricted the closer you get to them. I haven't played in a while so some of this may or may not be wrong, though I could try to answer more questions if asked ^_^