Not all creatures in "nature" have both parents present to raise the child they produce. In many situations the male bolts right after the lady is impregnated. I also think that I would note back that if it is only natural that there are a man and a woman involved in a child raising situation then it therefore should be only natural to have sex in order to procreate. Any sexual engagement that is not for the pointed purposes of having a child is not natural. That "naturally" men would steer clear of any woman that could not breed. There are some deeper incentives in that realm of thought -- thing is, as you put it, we've deviated so far from what could be considered a genetic purpose in so many ways that its hard to single out one particular thing that we still "should" be doing.

I also consider plastic surgery for vanity completely unnatural but it is quite amazing how many recipients of that like to tell me what is normal/natural and what not.

--k