But would that suggest that a man and woman being infertile is nature's way of preventing them from having children? The truth is, most children who are adopted are adopted by people that, for some reason or another, can't conceive children on their own. If your argument is that nature has determined their biology to be such that they can't conceive, that would imply that the only people fit to adopt children are those who can conceive.
You also can't say for sure whether a child being raised by two parents of the same gender would have negative consequences. There's no proof that it would start a "vicious, unhealthy cycle." As I said before, children raised by one parent tend to manage fine on their own with regard to gender influences.