First off, the analogy is crap. Mildly amusing, still crap.
Why is there a double standard? Because society is extremely sexist on the issue. I'd imagine it comes from treating women as property for thousands of years, coupled with some genetic underpinnings. The only way to be absolutely sure a child is fathered by a given man he must be the only person sleeping with the child's mother. Thus to ensure ones genes are spread, men seek to exert complete control over women's sexual activity. One easy way of doing so is a social stigma against loose women.
It was probably a stabilizing factor on society long ago as it would prevent fights over paternity, inheritance and such. Nowadays any positive function it may have had is long outdated yet it remains part of the social psyche due almost entirely to religion keeping it going.
