Semen can also upset your vaginal pH balance. A healthy vagina is normally acidic, while healthy sperm are alkaline, which can throw off the balance of bacteria in your vagina.
Unprotected penetrative sex with a man changes the pH of your vagina because semen has a pH of between 7.1 and 8. During sex, the pH of your vagina rises in order to create a more alkaline environment to protect the sperm and encourage fertilization. This can also encourage the growth of certain bacteria.