The story I heard goes like this. Once upon a time in ancient Nepal, society became really orthodox and sex became a sin. As a result people stopped having sex. And abstinence meant lesser babies. So the population dwindled. Less chidlren meant less people to work in the fertile fields of Kathmandu. Agricultural production went down. Things got bad.Then someone in the King's court came up with a idea. He said, what if we show pictures of the gods having sex - maybe people will not think so badly of sex. That's apparently how it all started re. The rest, as they say, is history. :)