Mommy - Where is Penis?

This weekend my son made a powerful discovery - boys and girls are different. At two-and-a-half, the only interest he's shown in my naked body before was when he decided in the shower that I had buttons on my chest. But this weekend, he asked a million dollar question.

I was going to the bathroom when he decided to join me. Most moms know that when you have a child you forfeit your right to potty in peace for many years to come. While I was finishing up, he cocked his head sideways and took a closer look.

"Mommy!" He pointed. "Where is penis?"

Hmmmmmmm.

"Well," I said, "I'm a girl and girls don't have penises."

I wasn't going to provide any more information than he needed.

"James have penis," he declared, mentioning his best friend by name.

"Yes, he does," I replied, washing my hands.

"Madeline have..."

Uh oh. I decided to wait him out and see what, in fact, he thought his friend Madeline had.

"Madeline have belly button," he said.

Thank goodness!

"Yes, Madeline has a belly button. But Madeline does not have a penis." I wanted to see if he had really learned anything.

"Do you know why Madeline doesn't have a penis," I asked?

No answer. Darn.

"Because she's a girl," I said. And off we went to play with trains.

So my son has noticed that our bodies are different. Does this mean I should put a stop to him seeing me naked? Because really, that's going to be a lot of work. To be honest, I'm a naked kind of girl. I mean, I'm no exhibitionist, but if I'm naked in one room and the clothes I want to wear are somewhere else, I'm not finding something to put on to walk from here to there (unless we have guests, of course). I'm not planning on visiting a nudist colony and I don't have any need to be naked in front of people, but I find being naked a comfortable condition (unlike my husband, who is without clothing as little as possible).

It's also very handy to be able to shower with a child. My husband has been working late a lot and it's helpful if my son and I can get our baths done at the same time. Otherwise I wind up washing my hair at 10 PM half the time because my son is having trouble getting to sleep.

At what point do I need to stop this behavior? When will it become uncomfortable (or otherwise negative) for my son to be in this situation? Until I figure it out I'll just try to be prepared for the next million dollar question - whatever it may be!