So, now we are supposed to define normal behavior by the way that a wild animal acts? Besides the scientific and religious problems with the theory of evolution this is another negative effect of the acceptance of Darwinism. It can be used to justify any behavior. I have even heard evolution used as a justification / explanation for rape. So, please spare me your attempt to drag humans down to the lowest behavior level of animals.
You haven’t been around much. Human beings all over the planet engage in voluntary sexual activity beginning around the time of puberty. Of course you don’t have to approve of that, they’ll do it anyway.