A person put up this rather interesting question on Quora –
How do actors handle themselves during sex/intimate scenes?
Here are some answers people gave (sourced from various interviews, quotes and articles)
It’s definitely not unheard of to get an erection while filming but I’ve heard of quite a few instances where the actor either says that they had tape to hold ‘it’ down or some sort of sleeve thing that attached to their leg. Some also say that its actually quite hard to get aroused because you’re surrounded by camera men and lighting and so many people it just makes it uncomfortable.
“Sex scenes can be quite awkward. As a guy, the first thing you want to do is make sure you’re not taking advantage. You don’t want the girl to feel like you’re getting a free feel or something. I try to make a fool of myself in one way or another to lighten the mood and then just go for it, because you don’t want to be doing take after take.” – Michael Fassbender, on Jane Eyre
“It’s hard to have a sex scene, period. It doesn’t matter if it’s a friend, a male, a female. You’re with 100-something crew members, lighting you, re-positioning you, there’s no comfort whatsoever.” – Mila Kunis, on Black Swan
For some actors/actresses it’s the most uncomfortable part of a shoot. I heard Cybil Sheppard say she hated the bed scenes with Bruce Willis. She said he used to fart right at the wrong moment and that would embarrass her. He would make comments and tickle her under the covers. But it all made for great fun when they did the editing.
I’ve never seen an actor with an erection, in all of the films and the sex scenes that I’ve done, but it doesn’t mean that I haven’t seen a guy being excited. It’s so technical, but we’re humans, and they’re naked, and they touch each other.
The scene wouldn’t be working if they weren’t. Obviously, you’re not literally doing penetration but they’re both aroused; they would have to be; otherwise the scene would be hopeless. On Fatal Attraction, Michael Douglas and Glenn Close were loosened up with Champagne and margaritas.
You can read the entire article here.