"I did a educational movie on the subject with Dr. Ellen Wiebe in Vancouver BC. Female ejaculate is of a different chemistry composition than pee, but if your bladder is full pee can be mixed with it when it squirts out.
http://www.hotmovies.com/studio/1311/Cream-Productions/
The skene's glands which are attached to the urethra, swell and fill up with female ejaculate and either gush or squirt fluid out. However women do not have a valve that stops pee from coming out of the urethra when cum is squirting out, like a man's anatomy does.
Therefore both pee and cum can squirt out at the same time."
To add to this... a part of My movie which was not added to the DVD, was that I had an internal ultra sound which showed that my bladder was empty before I squirted, when I squirted just cum. And as I said the cum was a different chemistry composition than my pee.
I am not making this up due to being desperate in any stretch of your imagination. Most of My clients are BDSM clients and I do quite well. Just because some women can't squirt, some women fake squirting, and some men think it's an urban myth etc etc... does not mean there is no such thing as female ejaculation.