How come i get utis after sex?

Bacteria milked up: There are bacteria from the rectum outside and around the urethral opening, which is only 1-2 millimeters away fro the vaginal opening. They get pushed up the urethra by the pumping action of penis in the vagina, which lies directly under the urethra. Thus bacteria are "massaged", pushed or milked up into the bladder where they multiply and cause a uti. Hence advice to empty bladder after sex.