Its only happened to me once, and it was very embarrassing. A girl had to be on top of me, she had spectacular breasts, and I hadnt rearranged my stuff into a harmless position. Shes basically rubbing herself all over me and, um, it got a bit hard. I had to apologize profusely afterward. Its not great when youre in a professional acting environment and somebody gets a boner, is it? No, not acceptable. You dont think of sex scenes as showing your bum to the nation. Its actually acutely uncomfortable being naked in a roomful of people. The very last thing it is is sexy. The actual physicality is very uncomfortable. All youre doing is smacking your nuts against someone, and nothing is going in.