Marriage is a concept founded on the nature of a man & woman being joined together as one (hence the stuff read before they are allowed to wed). Mother nature is a human made being and God is the creator of all. You must know that animals and humans cannot be compared when it comes to civilized occurrences. The difference between humans & animals is our minds & thus through thought, we are able to interpret civilized things. Whereas animals do not have minds & therefore cannot make civilized decisions as humans can in the context of choice. Contrary to your belief, a woman is distinct from a man by way of her womb and thus it is perverted for two beings of the same nature to be interwined.