Like jenk said it is absolutley to prevent infection. If you did get an infection it could permenantly harm your fertility. Trust me it is worth waiting it out. I know how you feel 100% as my twins are gravely ill and will likely pass in utero. Just be good to yourself and look at the big picture. You will get pregnant again and go on to have healthy children. Focus on that.
Good luck and I am very sorry for your loss.
It is to prevent infection. I was actually told after my d&c in December to wait FOUR weeks for sex and not to even have a bath for four weeks, too. That being said, we only waited about 8 days, which was when I stopped bleeding.
BTW, I am very sorry for your loss. I know how devastated you must feel. ((hugs))