According to a recent study from a medical journal.
"Erotic dreams were unrelated to sleeping on one’s back or side; however, they were linked to sleeping on one’s stomach. Specifically, the more frequently people slept on their stomachs, the more likely they were to report dreams about sex and about being nude. Incidentally, sleeping on one’s stomach was also related to dreams of being locked up or tied up, being smothered, as well as seeing UFOs."
I would guess that would explain a lot for me ... I sleep a lot on my back and side, and I rarely have sexual dreams. But when I do sleep on my stomach this is what my dreams be lookin' like LOL!