Premarital sex

Premarital sex (or nonmarital sex) is what happens when people who have not yet married make beautiful love. Generally speaking, the human sex drive is so strong (at least for most people) that it happens. It has always happened and will always happen. It happens because people seem incapable of controlling themselves or their desire, which leads to premarital sex and the inevitable destruction of the family due to deadbeat fathers and mothers who cannot support children. A great deal of poverty in developed nations can be deemed the result of premarital sex because people are not taught self-control and consequences.

Social opposition to and legal restrictions on extra-marital sexual activity are a typical factor in most human societies, but are especially pronounced in societies where religious justifications for rules of conduct for sexual relationships and the family unit are prominent.

Historical attitudes
In Britain, before the 20th century, young unmarried girls from the upper and middle classes were not allowed out without a chaperone. Nothing less drastic could protect their virtue until they became the property of their husbands. Upper class boys were allowed out on their own, and were tacitly expected to get sexual experience with lower class women and respect the purity of girls from their own class. In Europe and America, unmarried girls who became pregnant were often turned out of their family home without support. That discouraged the practice further.

Then birth control came along and clouded the whole picture.

Stopping premarital sex?
Without drastic measures, premarital sex will always happen. Teaching "abstinence only" doesn’t work. When young people have the freedom to go out and meet attractive partners (or unattractive partners, with the addition of alcohol), sex happens. Abstinence only education just ensures a high rate of extramarital pregnancy and/or abortion, since young people who can't contain their urges won't know how to satisfy said urges safely. (The fact that abstinence-only groups also tend to frown upon another safe outlet for sexual urges doesn't help matters.) More importantly, the idea that sex must be saved for marriage assumes that marriage is a good option for most people, and that sex is somehow "dirty" or "bad" outside of marriage.

Biblical view
The Bible says that any unmarried person indulging in sex is guilty of the sin of fornication, for which the penalty is death by stoning. Sex with a married person except one's own spouse is either adultery or fornication, for which the penalty is death by stoning. A woman who is not a virgin at her wedding may be killed by her husband, preferably by stoning.

The Bible is really against premarital sex, and really into getting stoned.

Shakespeare had it right
Escalus: How would you live, Pompey? by being a bawd? What do you think of the trade, Pompey? is it a lawful trade?

Pompey: If the law would allow it, sir.

Escalus: But the law will not allow it, Pompey; nor it shall not be allowed in Vienna.

Pompey: Does your worship mean to geld and splay all the youth of the city?

Escalus: No, Pompey.

Pompey: Truly, sir, in my poor opinion, they will to't then.

--Measure for Measure, Act II Sc. 1