No doubt the other 20% are of naked children for those of pedophile preference -*gag*.
It's a sad fact of life that such things exist, but let's sincerely hope the figure is considerably less than 20%.
As for the exploitation aspect, while womens forms have been exploited by movie makers, advertisers and magazine/newspaper editors, women know full well what men like and some purposely exploit that by posing/acting seductively. So iIt cuts both ways, doesn't it?
Furthermore, there are dozens of magazines and web sites that cater to womens desires and portray naked men. Again, while it could be said that these men are being exploited, these men also know women want, need. like these images and therefore exploit their weaknesses.
Personally, I think nudity is over-rated.... 80% indeed. If you've seen one boob you've seen them all... but then, variety (all different shapes and sizes) is the spice of life. 
