A social problem, answered using economics... I don't get it, but it does make sense while reading the article, and the reason why it makes sense is because of the constraints placed by the status quo. Even this blog had a go at it...

The Evolution of Human Relations
"From a technical standpoint, it is now possible to combine any mating pattern from strict monogamy to complete promiscuity with assured paternity. How many of those options actually go into common use will depend, among other things, on how much of our sexual behavior is hardwired and at what level."

I couldn't find where I read the bit about consenting adults, but that's the safe libertarian view I'm sticking to. In case you're wondering what all the fuss is about, it's because of this HBO show.