Well, you got me curious. Typing 'is urine sterile', google suggested 'is urine sterile as it passes from your body'. First result basically says that it's pretty much sterile unless you have some sort of infection. The thing is, I'd assume that any pathogen that comes from your body (however unlikely) would not be something that is contagious to plants or organisms in the soil anyway, and the pathogen would just die off without a host (another human).
Now, if you somehow got some plant or soil microbe disease stuck in your penis, then you might need to worry about passing it on. Anyway, like I said, I have no stake in this, so I'll just leave it at agreeing to disagree. You think it could be dangerous to microbes or plants and I don't. Even so, I'm going to stick with seaweed, earthworm poop, and bat crap in my tea... no piss for me.