I would think that it would be very hard to get UTIs from dirty diapers, especially in males. The male urinary tract (or any, really) isn't really built in a way that lets the non-sterile urine get back into it. I also don't see it happening much more with females (as it'd be hard for the urine to flow back up the sphincters in anyone, for one... they're designed to make it all go the other way about, see?)
That's not to say that staying in dirty diapers won't give you a potentially large amount of problems.


