From the "Boy, was Heinlein right!" deparment:
"Everybody lies about sex."
Why? Well, have you got a semester or two to discuss the sociological, psychological, and theological reasons? I'm beginning to think that there's a biological reason, too. As in, something physical within ourselves actually makes humans lie, when the topic is sex. And the sad thing is, it's the lying more than the sex that causes more disarray. "...It was he coverup."
Wanna see that dynamic go into hyperdrive? Check out the way adolescents deal with it.
I have never heard such of a pack of lies before in my life.
One of my biggest objections to drug use these days is that its illicit nature turns users into liars. This seems to be that natural order of things-- prohibited activity requires lying. Adolescents in decent homes in our society are prohibited from engaging in this activity. Take, say, a 14 year old and confront him with his father present about having sex, and he may very well deny that such an activity exists.
The funniest scene in the horrible old cult favorite Weird Science is when Kelly LeBrock (yum) confronts one of the teenaged boys' parents with a discussion of his solo sexual experience, with the boy literally screaming his feeble denials.
But at least the kids have an excuse.