Having a mental block has been mentioned on more than one thread and I think that is a very real thing for a woman. It takes a lot to get past that feeling of 'I'm gonna pee', so the natural thing would be to hold back, I know I've done that before when I've been with a guy, and know now that the feeling wasn't due to me needing to pee. I have still only ejaculated when masturbating on my own but that's due to lack of partner rather than trying and not getting there.
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