So MC, by your definition, then, oral sex is un-natural as well.

May I also suggest, that by your definition, heterosexual sex while wearing a condom is un-natural, since the penis is not being used for one of the two natural purposes you mention, but merely for pleasure?

And are either of the above any less un-natural than homosexual sex?

And if they're all un-natural, which, if one accepts your definition, they are, are any of the three any more wrong or less wrong than the others?

Or is it possible that there are three natural uses for the penis:

1) The elimination of liquid waste,
2) Procreation, and
3) To provide physical pleasure

That would seem to me to be quite likely, since, after all, sex is fun.