Maggie Gyllenhaal was interviewed by the New York Times about her role in the new movie Hysteria, and when asked why sex scenes are so difficult to tackle in a film, she said it's all about the reality of it:
[T]he most interesting sex scenes that I've done or seen are the ones that are truthful from a women's perspective — instead of what I think everybody got used to in the '80s and '90s: put on a black Victoria's Secret demi bra and be lit perfectly and arch your back. That's supposed to look like sex. But that doesn't look like sex for most people, and if it does, I think you're probably missing out on a lot. The more truthful you can be, the sexier it is and the more uncomfortable it can make you sitting next to a stranger in a movie theater.
Of course, things that are far-fetched and wild can make you just as uncomfortable in a theater, but she makes a very good point.
Tribeca: Maggie Gyllenhaal on Sex Scenes From a Woman's Perspective [New York Times]