What causes a mans urine to start smelling really bad?

They don't. It's not just one gender as that would be plain sexist. It is because of the sweat that gathers on their body and that releases a smell. The human body releases sweat for cooling the body, and that sweat also contains toxins, bacteria, and hormone metabolites. If boys smell more, it is probably because they play more sports like football and sweat more. However girls cover it up with deodorant. So in fact, anyone can have body odor.