That is the $64,000 question.
Honestly, I do not know. I'm sure that what Hawaiibelle said has a lot to do with it. But I also think that it is because sex is truly the one and only "intimate and personal bond" (or whatever you want to call it) that two people can share. Nothing else is really private or sacred.
I would have to agree that cheap sex personified (and glorified) by TV and the movies (and all the conformity whores that patronize them) is an obscene abomination.
That does not mean that I am opposed to sex by any stretch of the imagination. Quite frankly, I can't think of anything (except for maybe a good airplane photo shoot) that makes you feel better than by being intimate with THE PERSON YOU LOVE. Sure, your one night stands that you can harvest from your local bar may be good for a quick and momentary thrill, but in the end, there is no meaning, and with time and repetition, you eventually become spritually and mentally empty. I speak from experience on this.
I've had my share of one night flings. Did I enjoy them?
At the time, yes.
Am I proud of it?
Did I gain any long term gratification?
Again, a resounding no.
Nothing beats monogamy with someone who you truly care about. There is no shame. No guilt. No worries. No head games. If two people can be together, and only with each other, I think that is the best thing in the world.