That's the simple answer to a question posed to me by a friend. In brief, she was asking how and when sex became “bad” in Christianity. She asked it in the terms of a Frenchwoman brought up Catholic by nuns, but now lapsed.
(This is part of the interlude in the Story of the Faith series, which I will resume soon.)
There is no magical moment when ideas and new thinking suddenly appear. Trumpeteers did not appear at the gates of Rome on 476 to proclaim “Hear ye, hear ye, the Dark Ages have begun!”
The 18th century French did not invent human rights, there were several versions of the combustion engine before Diesel’s became accepted and several experimenters in long distance communication before Bell hit upon a particular approach and made money off it.
Ideas become accepted when a majority of people are ready to accept them. This happens slowly and once it does, everybody thinks it was always so or so obvious that everybody before them should have known.
The notion of separate and distinct spiritual and material things is from ancient Greece, which is the social culture that, as Nietzsche wisely put it, is the archetype of all European thought. The atom, democracy and homosexuality were all Greek. (They may have gotten it from someone else, but not that we know.)
The Jews thought differently. They did not separate the abstract from the concrete, spirit and matter, emotions from sex, etc. They saw them as all wrapped up together and saw no conflict among them.
This is why ancient Hebrew did not develop words for things that were not concrete and physically evident. The word for “spirit” in the Hebrew texts of the Bible is ruach (the transliterated ch pronounced as the Spanish j), meaning wind. People who were alive had wind inside them and breathed; people who were dead did not. If you separated spirit from matter, there was no person, only a corpse.
Jewish rules concerning sexuality were derived from Sumerian property law and a food etiquette whose origins are disputed.
Whose woman is this and who may sexually penetrate her at the risk of impregnating her and producing heirs? Keep in mind that no one knew the ovum existed until 1836. A woman was thought of as a hollow vessel into which a man inserted the seed of a human being.
Masturbation (onanism) was a sin in the Bible because it spilled seed, which a very small and besieged nation needed desperately to grow in numbers and to be able to survive.
This was not a sex morality of the purity and sanctity of virginity or chastity, but a social order imposed by survival, based on what was known at the time. It only was when Jews who followed the Nazarene preacher encountered Greeks head-on that sex became a moral issue.
Greek ideas about sex were pretty much this: have as much as you can in as great a variation as possible (if you are a free—or non-slave—man). Sexual passion and orgasm was come from the goddess Eros. In her temples priest-prostitutes, not always female, engaged in sexual worship.
The Greek attitude completely shocked the strictly monogamous, family-centered, reproduction-minded Jews. Paul writes no, no, no, no, no to the Greeks, outlawing it all.
But this was not Paul alone. This was surely the attitude of the majority of Christians, at the time Jewish. (There is evidence that, initially, the Romans could not tell Christians and Jews apart; they were all Jews to them.)
In part, this was caused by Greek misinterpretations of the Christian gospel. There was a very early heresy, I forget its name, that as part of the Eucharist included masturbation and the consumption of semen. Obviously, you can see why the apostles were just a little shocked.
When the Christian Jews who were missionaries got to Rome, a different reaction occurred among the wives of the householders who offered them a place to meet and celebrate.
“Aha! This Paul (and his friends) talk about it being better to marry than burn,” they thought. “He understands our slavery as Roman daughters and wives. We will oppose our fathers and our husbands and refuse sex in the name of this new god, Chrestus [an actual historical misspelling].”
Thus chastity was launched as a Christian virtue. Most notable is the case of St. Cecilia, who gouged her eyes out to be repulsive to the young man her father had groomed to be her husband. She lived the life of a virgin thereafter, a model to many; there were many others like her.
Priestly celibacy, which is not about sex but about marriage, came later. Around the year 1000, to end medieval nepotism in the Church at a time in which the clergy were the either the learned men at the nascent universities or, as bishops, feudal lords. They naturally helped their sons along in their clerical careers. Celibacy put an end to it.
Chastity, poverty and obedience were introduced as monastic vows in the 6th century by the Rule of St. Benedict, which attempted to bring order to the freelance and anarchic asceticism of the Desert Fathers.
My French friend was probably taught about virginity, celibacy and the monastic vows as one package handed down in special secret tablets by Jesus. Her teachers were women living under those vows and didn't really know any better. The good nuns were also suffering sexual repression (the true stories of wanton sex by former nuns are legion!).
Moreover, to women sex is often of overarching importance, as it can affect their social station in life and pregnancy can tether them. To men, free from pregnancy and long the socioeconomic patriarchs, sex is merely one more fun thing to do. Hence young girls were cautioned against sex by older women from time immemorial.
All that is how sex became “bad,” especially for pious Christian women. It's up to us to rescue both sex and real Christianity from the mix up.