I think a definition of "natural" is in order here. There seems to be variety.

The word natural is derived from nature, correct?

I believe my penis was designed for two functions; elimination of liquid waste and procreation. That's the nature of the penis. To perform the procreation part it contains very sensitive nerves and lots of blood vessels.

The rectum was designed for elimination of solid waste. No sex organs in there.

I think you see where this is leading.