I find it highly unnatural
No. Actually, inter-species sex happens. Have you ever seen a dog hump his owner's leg? I babysat a German Shepherd once that tried it with our puzzled cat. (Oh, the look on that cat's face was worth a million bucks!) Have you ever seen a horse mount a donkey? Humans are natural; as such, humans "doing it" with another species (for whatever reason and/or need) makes it natural. Okay, it's not common... (until you watch some of those "Debbie Does Dallas" videos where she's visiting a ranch. Horses and dogs seem to be the usual.)
Yeah, it disgusts me, too. So does human male to male sex. But that doesn't make it unnatural. Technically (aside from colloquial semantics) everything in the universe is natural. I do understand your usage of the word, "unnatural", however.