Egad - a voice of reason and logic. Shush, you, no one wants to hear that.
The cruddies that live on the average unwashed body are more un-sterile than urine and as this poster stated, the chlorine in the pool is going to kill it all anyhow. No biggie.
It really has nothing to do with what is biologically reasonable, but more of the fact that we are a civilized and advanced species, there is no need for us to urinate in our recreational water. I don't care if the chlorine kills anything in your pee, a swimming pool is not, and should not be used as an extremely large toilet.