...and "natural law" does not mean what is in nature is law. Pedophilia is in nature. Perhaps there's even a gene that perdisposes someone to pedophilia. Might be in nature, but it's not natural.
Marriage is not natural either; it's a social convention. It serves a purpose re maintaining societal norms that benefit society as a whole. Just because it would be more convenient to certain individuals to marry someone of the same gender (or for 7 people of both sexes to be collectively married each to the other six) does not mean it is beneficial to society.