Hahaha! I drew it because it was there. After you've done so many nude studies, the context of what you're drawing falls into the background as you focus on seeing and rendering. I always think it's odd when people do nude studies that are censored, or they left those areas blank. I had a figure sculpture professor who once said, "Don't leave things out because you're embarassed to sculpt them, for then your sculpture will never be human. Also, your grade will go down."
But I know it bothers a lot of people, so I'll just leave it at that.