George told Jerry "He saw a pipe". Jerry replied "Different pipes lead to different places" George responded "They're all pipes"
Clearly Seinfeld was wrong, and George was right. If seinfeld knew anything about waste disposal, and he should given his phobia about germs, he would have realised that the chances of the pipes going to 'different places' is remote.
In most municipal sewerage systems black water (crap and piss) is not seperated from grey water (sinks, showers, dishwashers etc). They both get piped and treated to the same sewerage plant. If the gym where George pissed had a septic tank then again, black and grey water both ends up in the septic tank.
The only situation that pipes would 'lead to different places' is if the gym reused its grey water (such as showers). The Grey water might be used, for instance, to water plants. I find this scenario unlikely however even then the pissy water would enter the ground and be rendered safe via natural processes (just as it is in a septic tank). The grey water could be reused to flush toilets in which case it would end up in the same place as the black water.
The gym may use composting toilets, wanting to produce compost from peopels crap. Some such toilets seperate out piss and crap however I think this scenario is unlikely.
If seinfeld knew anything about sewerage he would agree that George pissing in the shower was totally acceptable.
rob