"Lyoll
Actually, very often people need scientific evidence to convince them that nature is natural . . . we're so comfortable in our cultural blinders."
'Natural' is that most flexible of arts.
Speaking of which, baby daddies leaving a trail of baby mamas on their own is something supported by most of the natural kingdom, minus the repeat appearances on Maury, natch.
The thing with all things 'natural' is that they have the curious tendency to have mechanisms to facilitate the behavior, not repurposed ones.
This is not to pick on homosexuality, as heterosexuality utilizes the same not-quite-often-exhibited-on-Wild-Kingdom array of felatio, sodomy, boob play, making out, and that all-important money shot. Oh, and gimp suits. Ain't many penguins with those on.
All things natural tend to have a purpose for the behavior, as somewhere between the endless chasing prey and dodging predators, not much calories left over for sexual gymnastics. Humans do cross this line, but it's part of pair bonding that will induce help from the mate in raising offspring instead of him leaving to sire more offspring with other females and so on. That's traditionally, not the more modern tendencies to engage in behaviors of ever more contorted and functionally useless behavior.
All things natural tend to be shared by the species as a whole, not just by a small segment. And thus being natural and common to all, there's no need to prove it's natural.
Perhaps focusing on proving that weird sexual behaviors that humans indulge in a lot because we no longer have to worry about lions pouncing on us and we got more calories coursing through our veins than we need is not the way to go. Hey, you might have a case for claiming unnatural behaviors are natural because so many do them nowadays, but the fact is perversion is perversion for everyone, despite the lovely tale of two lonely imprisoned male penguins in the zoo that sat on the same nest and clicked bills before a new female was introduced into the exhibit and that's when Harry met Sally and left Franky all alone on the nest THE END