Because if you do, you will hear the words "penis" and "vagina" way more than you ever bargained for.
You might think that having a six, three and two year old wouldn't lend itself to the necessity of saying penis and vagina over and over again. But my kids, especially the six and three year old seem to find a reason to say it almost every day, sometimes several times a day. A big starter for using these words is the fact that we only have 1 1/2 bathrooms in our house. While Ian is taking his bath, Emma will run in and use the bathroom. While Emma is washing her hands, Ian will run in to go. While I'm using the bathroom, all three kids will join me and discuss some scholarly topic like which show is better, the original Scooby Doo or the What's New Scooby Doo series. Consequently, obvious differences in anatomy and peeing methods is up for discussion on a regular basis.
It was the Ian brushing his teeth, Emma use the toilet scenario that sparked the flow of body part words I was subjected to this morning. I overheard Ian saying to Emma, "You have a vagina, Zoe has a vagina, Mom has a vagina, Keara has a vagina, Grace has a vagina." It was at this point that I told him to quit talking about people's body parts.
I think I have heard the words penis and vagina enough times that I could go the rest of my life without hearing them again. Unfortunately, that's not going to happen anytime soon.