Urine is sterile when it is first passed. However, bacteria grows on it pretty rapidly once it is out of the sterile urinary tract.
The smell of urine bothers me a bit (working in health care sometimes desensitizes you to things, and other times sensitizes you to things. I'm sensitized to the smell of stale urine and don't like my house to smell like it.)
I flush every time. We have a private well for our water system and live in an area with plenty of fresh water, so I am not concerned about it. Plus our pee and poo go into a septic system, where they break down and are pumped out once a year or so (in a much reduced form) by a septic company we hire to do this job. They usually incinerate the contents and... I don't know what they do with the ashes.