Urine in the bladder is sterile to a certain extent, but once it comes out of the bladder, it passes thru the urethra and picks up the microorganisms therein and innoculates the wound with the said bacteria.
Primitive cultures had no concept of microorganisms and sterility hence they should be forgiven for treating wounds as such.
But like you said, if the choice was to have crud in the wound and nothing else, I would choose blood. Let the wound bleed to clear out the crud, then stop the bleeding. It would certainly be better than pissing on it.... <img src="/images/graemlins/blush.gif" alt="" /> But of course, if you really have no other choice, with no other irrigation fluids available, this would be better than dirt.....

In terms of preference:
Sterile saline
Non sterile saline
Sterile water
Non sterile water
Clean water
Blood