Why do girls get bladder infections after sex?

Short urethra: The primary reason is a short distance between the outside of the urethra (where the urine comes out) and the bladder. The bladder is sterile (no bacteria normally), whereas the vagina (where the urethra exits from) is in the midst of billions of bacteria. The act of sex pushes skin cells and bacteria into the bladder. Urinating immediately afer sex is very important to avoid an infection.

Uti: Post coital utis are common. Formerly called "honeymooner's cystitis", these are generally caused by the normal colonization of bacteria surrounding the vagina and anus. With the short urethra of women and the trauma of intercourse, these can migrate into bladder and lead to a uti. Emptying the bladder before and after intercourse is the best prevention.