This is not based on any scientifical research (but you can look it up yourself) but I would say that the penis and penis head is very resistant against germns and bacterias.
Remember we weren't born clothed nor did our ancesters walk around clothed. We aren't naturally supposed to be protected by clothes against germs. Think about some of the so called primitive tribes we have in our world. They walk around almost, if not completely, naked and they don't seem to suffer from any kinds of infections in the genital area any more than we do.
Plus (and keep in mind that I am only writing this out of thinking, I haven't checked for any scientific proof for this), I believe that the genital tissue is very, very, very similar to our oral organs, namely the lips. For instance, the only areas of our body that don't have any hair on them are the lips, glans and vaginal labia, if I'm not mistaken - and this I have read somewhere, it's a scientific fact ;o). Also, when you get aroused, your lips will swallow, as happens with the penis and the pussy labia. So, if you think along thses lines, do you ever get infections on your lips? You probably don't, and they are much more exposed than your penis.

It's just a thought. I don't know if it can be backed up, I hope it can. Anyway, keeping a good hygene is always good ;o)

I hope this has helped.