Dude, when it happens naturally, women have no control over it. That is the meaning of the word "involuntary". Muscles loosen up during orgasm and some pee leaks out of the bladder, mixes with come, and shoots out. Honestly all that a woman really knows is that the stuff comes out, it took catheter experiments and chemical testing to determine that it was pee, no doubt most women don't believe it is--but whatever, people believe lots of things.they wouldn't be doing it if it was pee!
Just enjoy it.