Why is sex virtually taboo in some countries?

[Madame Adequate]

Even deeper than puritanism and other paradigms that have historically shunned public sexuality, I'd wager that there are two forces at work:
1) Sexuality is an intensely personal experience, one that makes an individual feel self-satisfied, and therefore its restriction and demonization benefit leaders who desire more devotion and less independence from their followers. Cf. 1984 by Orwell.
2) Sexuality can produce extremely volatile emotions, and so the stoic philosophy that has extended from sections of ancient Greece to modern authoritarian regimes and even large controlling groups in current non-authoritarian societies (e.g. religions) represses sexuality in order to promote the [silly] idea of happiness through emotional stability.

Oh, and by the way, Foucault is the man.

This is absolutely the best post on EoFF at this time. I support 100%.

Not taboo, nowadays it is even wise with all the STDs and AIDS. The less partners the better.

Though I'd recommend against waiting for a virgin to "start". They are a rarity nowadays so you may look for a looong time And when you find her / him you will have to marry the virgin. Don't even think of getting "it" then breaking up later. It is the cruelest thing ever to do to someone who reserved their first time for YOU. Only jerks do it.

Whilst I kind of agree with the second point, I disagree with the first one. Sex as a taboo doesn't stop sex, it merely stops discussion of sex, and it's pretty well-evidenced that people who grow up in societies which can have open discussion about it are broadly speaking sexually healthier than those who don't.