Get Religion has gathered some interesting stories on research on possible physical and genetic causes for pedophilia, that is, for a desire to have sex with children.
This is a scientific question, and I am unqualified to make any pronouncement on it.
But if pedophilia is genetically determined, and therefore “natural,” those who argue that any genetically determined sexual desire (for example, homosexuality) is therefore natural are going to be in a difficult position.
But philosophers have long known, and Christianity concurs, that our desires are more or less disordered. Some people have an inordinate craving for recognition, food, money, sex, inordinate both in the strength of desire and inordinate in directing the desire to the wrong object. Some criminals commit their crimes so that they will become famous.
Desire is not self-validating. Many of the arguments about “following one’s conscience” sound suspiciously like “I really, really want to do this, so it can’t be wrong.”