Wasn't this question answered in "When Harry Met Sally"? The real question is why?
Dr. John Grohol over at PsychCentral published this article yesterday: Why Women (And Men!) Fake Orgasm? The results of a study done at the University of Kansas (my alma mater) aren't exactly surprising:
that most of us follow, or would like to follow. Boy meets girl, girl
takes boy to bed, girl has an orgasm before the boy. And the boy [has an orgasm] in
response to the girl’s orgasm (although not as much, vice-a-versa).
Faking an orgasm is a predictable response to this set of expectations,
to ensure the “script” goes as smoothly as possible.
Read the article. And then enjoy the video. It's a hoot.
The older woman at the end of the clip is the director, Rob Reiner's mother, Carl Reiner's wife, Estelle. She practically steals the scene in less than two seconds.