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In my experience, combined with having read the famous sex research of Master's & Johnson (I think that was their names), after a certain period of time has elapsed, a girl who has had sex will become very slippery. In the study it says that semen within the vagina changes consistency after a while to a more slippery state (I don't think that was the technical term, but its been years since I read that). This helps the sperm swim around the vagina. Also there is a hollow shape to the structure of the vagina at the base of the uterus that can serve as a well to store the semen in order to aid in fertilization (being that it sits at the base of the uterus).
My wife and I have an open relationship in which she has a lover which she meets on occaision and after which she shares the experience in detail while we have sex, usually recreating the experience. It takes her about a half an hour to an hour to get home from his place, by which time his cum is very slippery. Also, in relation to the second scientific tidbit, the deeper I penetrate, the more of his cum is released. Sometimes when he does not produce much cum, it takes a few deep strokes to make her wet and slippery with his cum. In my own experience, the only other times that her own wetness compared to that kind of a feeling was a few times when she had a yeast, or some similar vaginal infection.
Mountain Valley Wolf