"Polone: Three Actors Reveal the Awkward Truth of Shooting Sex Scenes"

" Have you or the person with whom you’ve been doing a scene ever become, uh, aroused?

Betty: Not to mention any names, but one actor came on me during the take. I had to surreptitiously wipe myself off with the sheet. Fortunately, I liked the guy. I found it a little flattering and a little creepy. We never talked about it, so I can't tell you if it was Method acting on his part or if he just found me pretty, but I suspect I'm not the only actress who's had this experience. But I can tell you that twenty years later, when I run into him, my first thought is, There's the guy that had on-camera sex with my abdomen."
http://www.vulture.com/m/2012/03/shooting-a-sex-scene-polone.html

I have often wondered about this, and this article confirms all my suspicions about the awkward.