Do they really have sex in movies?

Most of them are not. They are just acting. The only time they actually do participate in it is in porn. Unless the camera shows you that the actress or actor is trying to "hide" something from the camera's view then you know that it is for pretend.

(Source: Wikianswers)

Usually they would use a body double for the nude parts in movies. But in normal film genres, the scenes are not real. It's just clever camera takes and film direction. It could take a day to film it!

Scenes may appear steamy, and there may be a lot of skin contact (one could argue that this is sexual activity) but there are barriers in place. Directors do attempt to be as sensitive to the needs of the performers as possible.

(Source: Wikianswers)

Actors have lessons on doing love scenes. There can be serious repercussions and allegations if it were for real. Here's are some excellent in-depth answers to question:

1. http://en.allexperts.com/q/Careers-Acting-Performing-1479/Love-Scenes.htm
2. http://blogs.smh.com.au/entertainment/archives/box_office/007549.html

Also see this answer - the part where this person said he or she has worked in the motion picture industry for 20 years for question 'How do actors not get aroused during sex scenes?'

(Source: Wikianswers)

Though there are rumors/facts who says some couples at movies have sex, not many do that, is usually simulation of sex.:

1. Wiki list of Films with non-simulated sexual activity
2. Leung Ka-Fai or Tony Leung and Jane March at "the Lover" (1992, France),
3. While filming "Boxcar Bertha"(1972), stars Barbara Hershey and David Carradine fell in love. They have said in interviews that some of their sex scenes were real. This film is directed by Martin Scorsese.

While standard movie technique is to suggest by clever positioning of the camera and careful scene setup that sex is taking place, there have always been rumours of more. These are hard to verify but plausible. For example, Last Tango in Paris was rumoured to have some real sex as was Don't Look Now.

But mainstream censorship rules in many countries would have prevented any possible verification of this in the released footage as, if the movie were explicit enough to see real sexual acts such as penetration, it would automatically receive a pornography classification.

But some censors have been getting a lot more relaxed recently. In 2004 the BBFC (the british film classification body) approved a mainstream release (18 certificate, so nobody can see it under the age of 18) of 9 Songs, a movie by the mainstream (if a bit arthouse) british director Michael Winterbottom. In 9 Songs the actors really did have sex on set and the movie is explicit enough to see this. The movie was controversial (see this Guardian story written before release) at the time, but less so than expected. More surprisingly, it isn't very erotic as the lack of subtlety in the sex scenes seems to neutralise their effect (for example, see this Guardian review).

What 9 Songs proves is that, sometimes, people really are having sex in the movie. But there is little correlation between the reality and the erotic power of the scenes.