What about lesbianism then? What about other animals and homosexual exclusiveness? What percentage of male sheep rape? What percentage of male sheep go after young sheep? How many male sheep do not have exclusive sexual partners? What percentage of male sheep behave any other undesirable way? Many bad behaviors have been with us from the beginning. Greed is natural. Lust is natural. That's no argument.
We can rise above animal impulses. Certainly some of us can. Age of consent varies by state and nation. Would the author also hold up studies that show that adults and minors should engage in sex (or that such activity should at least have a universally agreed age)?
Are sheep supposed to be perfect beings incapable of unatural behavior?
The sheep lie.