Okay....Urine *is* sterile WHEN IT IS IN THE BODY (bladder). Just like blood is sterile when it's in a vein or artery.
However, once it passes through the urethral meatus (pee-pee hole) IT IS NO LONGER STERILE.
why?
Because a whole shitload (literally, sometimes) of bugs and bacteria live down there. That's why women get so frequent UTI's because the urethra (tube connecting the pee-pee hole to the bladder) is very small compared to men, and that short urethra allows the bacteria that live on the outside to get inside the urethra (pee pee hole) and have happy hippy parties and such. This is why women (and men) should always urinate AFTER sex, because it helps to cleanse the urethra of all the bacteria that may have wiggled their way up there during sex---the urethra kind of takes a hit (heh) and gets a bit swollen, opens a bit more, and more bacteria can wiggle inside.
That is why such care has to be taken when inserting a urinary catheter as far as doing "sterile" technique, cleansing and prepping the urethra/labia/glans penis, because you want to get all those icky bugs away from there as much as possible. Of course, it is impossible to "sterilize" human skin, but we do the best we can via betadine solution and using sterile gloves and equipment.
I mean, the fact that people can even THINK that pee, outside of the body, I guess affirms why only 40% of people believe in Evolution. I mean, have you SEEN where pee comes from? Do you know what hole is located just a few short (heh) inches from the pee pee hole? Do you understand the conditions those parts are kept in? From the warm dark place, to the underwear, to the fact that guys don't even generally WIPE after they pee....the whole proximity of the anus is really only a consideration in females but COME ON. SERIOUSLY....you think something that comes out of the GENITAL AREA is sterile? oy. fucking. vey.
Once the pee passes out of the bladder, it's about as sterile as a big hot poop, which is to say 'not sterile at all'.
And I'd inject urine into my blood about as quickly as I'd inject hot poop into my blood, which is to say "never"
Here are some illustrative guides drawn by someone that was not me: