These are my reasons. It is just another part of the beauty of life, and nature. The problem was created by misconceptions, and solved by them as well. People believe that sex is a dirty act. If it is, then don't think for a second you'll ever be clean.
Half way is obviously close enough to attach the same stigmas to it.
Nudity is also very poorly defined. A breast can be in clear sight, but the nipple exposed is obscene. So petty, so foolish. If these people want to hide these aspects of life from their child's or their own eyes, let them take responsibility for that protection. The last thing that bothers me is that it mostly screws women.