Sexual intercourse can cause vaginitis, especially if you have an unclean sexual history. Because the penis enters the vagina during sex, bacteria and viruses from the penis can enter the vagina, leading to vaginitis. Changes in the vaginal hormone level during sex can cause changes in the pH of the vagina, which can lead to the reproduction of opportunistic pathogenic bacteria in the vagina, leading to vaginitis. Squeezing during sex can cause bacteria from the skin around the anus and buttocks to enter the vagina, which can also lead to vaginitis. Therefore, it is important to wash the penis and external female genitalia before and after sex to effectively reduce the probability of getting infected.