This is a myth. Urinating after intercourse will not prevent pregnancy, but it does reduce the chance that you'll get a urinary tract infection.

The semen and penis go into the vagina and the urine comes out the urethra, which is a different opening. So, while urinating after sex might empty the bladder, it has no effect on where the sperm are - inside the vagina. Once sperm are released into the vagina, they begin swimming fast up into a woman's body in search of an egg to fertilize.

To see a diagram of your vagina and the urethra click on:
vagina
urethra

You can also take a guided tour of your vulva on Scarleteen.com.