A Christian principle, adopted by St Augustine from the ancient Greeks, it that every part of nature has a proper function. This reasonable sounding proposition can be extended to a less reasonable conclusion: that every part of nature, and in particular every part of the human body, should be used for its proper function and for nothing else. This idea was still familiar to Christian believers into the twentieth century, generally to justify prohibition: If God had meant you to smoke, he would have given you a chimney. If God had intended you to swim, he would have given you fins. If God had intended you to fly, he would have given you wings. This sort of argument has largely been abandoned (applying it consistently takes theologians where they prefer not to go). But there is one example of this idea that is still applied almost as strongly as it was in the time of the Cathars. God had designed the sex organs for the purpose of reproduction, so it was and is wrong to use them for anything else. In particular it was, and is, of the utmost importance that semen should should be deposited in a human vagina. Every sperm is sacred.
This idea explained many aspects of Catholic theology which seem odd to outsiders. Not only did it justify bans on sodomy and contraception, but also coitus interuptus and masturbation. On this question, Cathars held almost exactly the opposite view. While Catholics taught that semen should be deposited where it could lead to conception, Cathars held that semen could be deposited anywhere that it could not lead to conception. So it was that on one hand practices like masturbation could be no sin whatsoever to Cathars, and why on the other Catholics could believe it to be a heinous crime against God. Who practised it more is a different question, and one to which we do not know the answer. (Catholic teachings following the traditional line of argument have now been abandoned, or at least are no longer openly advocated. For example, as we know from medieval penitentials, experiencing a nocturnal emission was a far more serious sin than committing rape. The former involved spilling seed outside its divinely appointed receptacle, and the latter involved depositing it in the correct receptacle. The former therefore was a serious sin, and the latter was not.)
Bestiality
