Yes women really do it, and yes it is very sexy. Every woman is capable of doing it, but they don't know how. It's from knowing how to work the muscles.
Ladies, if you want to learn, practice kegel's.
My ex did it every time we had sex. We too debated whether or not it was urine. We never came to a conclusion on the answer.
She once did it so much that she completely soaked my comforter. I couldn't wash it right away so I just threw it in the closet. A couple of days later when I was ready to wash it, it smelled like urine to me.
I looked it up online, and there are no consistent answers. It seems like half the medical community says it is urine and the other half says it isn't.
My ex never believed it was because no matter how many times I would make her squirt during sex, afterward, she would have to use the bathroom.
So in her mind it couldn't have been urine because she should have been empty, but to me, that's what it smelled like.
It even looked like it once when we had sex in my office, and it pooled up on the chair mat, it had a yellowish hue.
I think it's debate that will never be settled.