Because we are guys. It is really is simple as that. It is kind of the same concept of why girls go to the bathroom in pairs.

Cuz We're stinky!!

Not all mens loo's a stinky. In fact my wife will often comment on how stinky a ladies public loo is, if she's used one! Any public loo which isn't cleaned will stink.

Because of the venting from the women's

Didn't someone ever tell you, Men are, oink, pigs, oink?

To keep women (cootie-bearers) out. :o)

Because it is fun to pee on the floor.

Because of the urinals and because some guys don't flush.

I am not sure if they are always more stinky than women's restrooms. Here some reasons for lack of cleanliness in general: "Some high-capacity washrooms can be more modern and and clean, while others will be dirty and outdated. Sometimes, an attendant will be present in these washrooms to ensure cleanliness and assist users." "Generally speaking, a washroom at a more elegant restaurant would probably be more clean and comfortable than one at a fast food restaurant." "School toilets are almost always dirty, and some are only cleaned on a weekly basis. There is commonly barely any privacy offered in a school washroom. Often stall doors are torn off, or were never present. Walls of the cubicles are usually full of graffiti, and some are much shorter than a regular restroom's. The reasons for these problems are due to destruction by the teenagers who use them, and no funding from the schools to maintain the facilities. As a prank, a young male may leave feces in urinals, throw trash into toilets and swing from stall doors." "Many public washrooms around the world are generally dirty due to heavy traffic, and the lack of available housekeeping staff to keep up with the cleaning. Some private businesses prohibit non-patrons from using their facilities as public washrooms in order to reduce the amount of traffic and the amount of cleaning necessary. Some go as far as locking the doors and providing keys to patrons only. Toilets that require a pay on entry are usually cleaner than free toilets. Dr Dipak Chatterjee of Mumbai newspaper Daily News and Analysis claims that public toilet facilities are so unhygienic that people — especially women — who are vulnerable to infections should consider wearing adult diapers instead. Some cities, like Philadelphia, are launching major efforts to install dozens of high-tech, self-cleaning public pay toilets in their heaviest pedestrian and tourist areas. Jack Sim, founder of the World Toilet Organization, has campaigned internationally for better, cleaner public toilets, particularly in developing nations." Source and further information: http://en.wikipedia.org/wiki/Washroom

I never knew they were.

Depends on the establishment. I think men's rooms are not as frequently cleaned as women's rooms. Could be also some with bad aim, or not shaking it enough after going. (or shaking it haphazardly)

restrooms should have a vent at the door & exhaust vents in the ceiling to allow proper ventilation. If the exhaust vent isn't working right, that's the problem.