The normal bacteria in the vagina produce lactic acid, rendering the whole place acidic. I can't remember if this in itself has antiseptic properties or not.
I do know that semen is alkaline, and that sperm are killed by acid. The vagina is quite a hostile place for the wee armies. The fluids they swim in help to neutralize their new territory some, but many of them die anyway. Their carcasses make up a large part of wet spots.
Some vaginal bacteria suffer when their environmental pH is raised by the alkaline semen. This, and the possible presence of yeast and other microorganisms on the penis depositing the semen can lead to yeast or other infections in women who are susceptible to this. A lot of the time these kind of sex-induced infections are mild and go away on their own.
However, there is quite an industry built around pH and "freshness" restoring douches. I've never used any, but I've never had an issue with my vaginal pH or freshness. I've also had douches condemned to me by a doctor, who explained that the flow of liquid in the vagina can simply mix any infecting populations around and risk ushering them into the uterus itself, which is a much more serious infection to have.
Also, in case anyone hadn't figured it out, douching with an acid after unprotected sex will not kill all the sperm and prevent a woman from becoming pregnant. They will be swimming away, and are partly protected by their alkaline fluids. The liquid can sweep them into the unterus, and any acid not very dilute can do damage to the vagina. Thus ends my education of naive teens.
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