The link may simply be that the act is not normal. Because if it where normal the process, both human sexes would be able to create life. That is what you see every where in the animal kingdom. If an animal can have offspring it does it eith asexual or it is capable of having both sexs at the same time or it naturally changes to the other sex. The fact that only one sexes can do this naturaly ( i mean with without outside help) in humans. Making these acts unatural because humans natural instinct to procreate wich is what drives the sexual drive of humans. Making these acts unnatural.