I would like it if all of my girlfriends did this but i know realistically this is not going to happen, so just be happy with what you have and don't pressure her if she never does this. They have adult movies that you can purchase for this pleasure; one that come to mind is called oh my gush number #7.
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It probably is not urine- though some women do leak urine or even quite a bit of it when they have an orgasm. It may be ' female ejaculation' although I think that is not a real description of what happens. It is not an ejaculation- a consequence of orgasm that aids the intensity of feeling. Rather, it seems to be some of the same kind of liquid produced by a woman's lubrication. This can collect. for a minority of women, in pockets of some glands and shoots out when orgasm causes contractions.
This can be a lot of liquid or just a little- and other than using a towel, there doesn't seem to be any way to control it or modify the wetness. Its just natural for some women . There is some controversy , I should add, about what the liquid is, and how the mechanics work-- this is just the version that I have come to read in the medical books that seems the most reasonable explanation to me.