Because some of the others mums think I am 
I don't like all the other names that are bandied around. If there was a 'softer' word, like the equivalent for willy, I would use that but there only seems to be words like daisy or minnie that don't actually indicate what you're talking about.
I know the 'what do you call it' question gets asked a lot, but AIBU to say vagina? Other mums think it is very weird/rude that DD now says "'gina"...