Naturalness

I've had two good friends in my life who were gay. Both came out of the closet when they were adults, married, and with children. Both formed stable relationships, in one case, a life-long one. In neither case did I suspect that they were gay until they came out.
I mention this because of one of the arguments I hear against homosexuality: It isn't natural.
I accepted this argument, more or less, for a lotta years, until I asked myself, "Who defines what's natural or not?"
Turns out it's not Mother Nature, because there are numerous examples of homosexuality throughout the animal kingdom (Heck, for all I know, there are gay plants).
It's the keepers of the moral flame who have decided it's not natural. The argument is an old one, and is found in various religious texts all over the world. It's also set down in musty law books as "The dreadful crime against nature." Reminds me of the saying (I wish I know who said it): "There are two kinds of people: The righteous and the unrighteous. The classifying is done by the righteous."
However, since nature herself doesn't seem to have any objections, I don't see why I should. In fact, I think I may even be able to see it as a good thing. For one thing, it cuts down on the competition for the girls.