To be fair, though, Christianity got this idea from Plato via Augustine. Plato's idea, as expressed for example in the Phaedo, was not so much that sex is bad, but that anything having to do with the body or corporeal nature in general is bad. It's bad, he thought, because all such things are impermanent; and the purpose of life is to rise about the corporeal and contemplate the eternal and unchanging Forms. The problem, according to Plato, is that when one tries to contemplate the eternal, the body is a constant distraction. For example, someone might be minding his own business contemplating the Form of circularity, but then a beautiful girl walks by. ... So much for circularity.
Originally Posted by blansky
All of this is especially funny because Augustine was quite the Lothario.