If we take as given that the human sex drive is primarily, from a biologial point of view, focussed on re-production, therefore sexual attraction is designed to be the seeking out of a suitable mate with whom to produce offspring.

If a person's overwhelming sexual desire is for someone (or something for that matter!) with whom they would not be able to produce a child, then a further explanation is required.

But the idea that the human sex drive is primarily focussed on reproduction is hardly an established scientific fact. It's widely accepted a priori, but there is nothing to prove this. Even if the "reason" for the existance of the drive were solely for reproduction, the drive itself is not focussed on reproduction, by which I mean, men are not aroused by thoughts of reproduction, or contemplating reproduction when mounting a woman. He's interested in "getting some," not producing offspring, and in "coming," not fertilizing.

If you are not convinced, then why does a man find a woman attractive even if he knows she's not fertile (e.g., tubes tied)? Why would ANYBODY find post-menopausal women attractive?

Student