Random question

Why do I always hit writers' block when I get to the sex scenes?

I mean, seriously. I'm 12000 words into this story, I finally get to this crucial scene and...pfft! It's gone. Suddenly I'm tidying my room, sorting through DVDs, mixing a drink - anything to not go back to this fic. Damn, girl, anyone would think I'm scared of sex. It doesn't even need to be especially graphic...