Early Christian beliefs about sex might not seem surprising at first: intercourse was generally seen as evil, and it was only acceptable after marriage. But dig a little deeper into the rules for early Christianity, and you’ll find all kinds of weird stuff.
When it came to Christianity and premarital intimacy, the rules were strict—and sometimes they were just as strict once you were married. Doing it on the wrong day or in the wrong position made even marital intercourse a sin, according to early Christianity.
Medieval Christianity wasn’t as buttoned-up as some people believe. And some things haven’t changed much since the Middle Ages. As for adultery in the Middle Ages, who knew that sleeping with your wife could be adulterous? And that’s only the beginning—wait until you see what early Christians called “the worst evil.”
Certain sexual sins in early Christianity were pretty straightforward: no intercourse before marriage. No inter-familial relationships. No rape. And definitely not whatever’s happening in this 14th-century manuscript.
But other sexual sins are pretty weird by modern standards. Many of them boil down to one principle, enshrined in early Christianity: intercourse and lust were polluting, and they should be avoided if at all possible. Saint Paul recommended celibacy, but he agreed that those who could not "contain themselves" should find a spouse, "for it is better to marry than to be burnt." That's right: lust sent you straight to Hell.
If you just had to do it, it could only occur under very specific conditions. And any kind of intercourse that could not lead to procreation was a sin.
The early Christian church went out of its way to limit intercourse—even between married couples. Religious laws and proclamations issued by the Catholic Church tried to restrict when people could sleep with each other, and they ended up banning most of the year. No "happy time" on Sundays, one rule proclaimed. That was the Lord’s Day, and it shouldn’t be contaminated by lust. And don’t plan on doing it on Thursdays or Fridays, either, because those days should be spent preparing for communion.
Intercourse was also banned during Lent, before Christmas, and at the Feast of Pentecost, which together added over five months during which it was disallowed. On top of that, early Christians were not supposed to have relations on Feast Days. In the early church, you pretty much had to check your calendar before getting it on.
Never heard of intercrural intercourse? Don’t worry, it’s a sin according to early Christians. Intercrural intercourse was “copulation between the thighs.” This sort of thigh rubbing to completion was an “act against nature,” according to the church—even if it occurred between husband and wife. But intercrural intercourse was more often associated with same-gender practices, which the church found problematic because they could not lead to conception.
Thomas Aquinas labeled all sexual activity besides vaginal intercourse “sodomy,” regardless of the gender of the people involved, because those sexual acts could not cause pregnancy.
There is one evil, an evil above all other evil, that I am aware is always with me, that grievously and piteously lacerates and afflicts my soul. It was with me from the cradle, it grew with me in childhood, in adolescences, in my youth it always struck me, and it does not desert me even now that my limbs are failing because of my old age. This evil is sexual desire.
Anselm described it as “the storm of lust that has smashed and battered my unhappy soul, emptied it of all strength, and left it weak and empty.” So yes, even thoughts could be sexual sins to early Christians.