It is my understanding that urinating after sex can clear the urinary tract of bacteria that may have entered, therefore reducing the likelyhood of a UTI.

Yeast infections tend to be the result of overgrowth of yeast located in the vagina. This can happen for a variety of reasons, but simply having sex (unless it's with someone else who has a severe yeast infection) does not necessarily increase the risk of yeast overgrowth.

If you are concerned about any of these things because you have symptoms, I would recommend contacting your doctor for advice on how to treat and prevent these conditions.

Well, first I would make sure that both or all people involved had taken a recent bath or shower to lessen the growth of bacteria, and also clean up after. I could see how urinating might decrease your risk, but I'm sure it won't prevent an infection. I second what Laurel says, talk to your doctor.