When I write sexual scenes (I don't write erotica), I try to put myself in the moment, think about the point-of-view character, and try to imagine what he or she would be thinking, feeling, doing in that moment. Of course, the reader doesn't need to see every little detailed movement, just the bigger actions and thoughts that would be on the character's mind. Is she nervous? Excited? Worried about something? Or completely caught up in the moment? Is it pure sex or mad love? Is the character trying to prove something to his partner, or impress her, or please her? Is she focused on her own needs or trying to make a positive experience for him? And so on...
Anyway, that is what works for me, and I've been told my sex scenes are well done. It's especially fun for me when I write from the man's perspective and have to try to think the way a man would. At least one male reader told me I did indeed have a keen understanding of the way the male mind operates. To myself I thought, "Duh--like men are so hard to figure out!" LOL.
Maria