In a society like Japan's, where civility in public requires a high degree of decorous formality, crude language rarely rears its ugly head.
Japanese is, in fact, rich in four-letter words, slang and argot, but such expressions are not normally encountered in public intercourse. One of my slang dictionaries has four pages of entries under "vagina"; and well might you ask who is actually using these words in the first place.
The answer to that question holds the key to understanding the role that "dirty" words play in Japanese society. It is only the rough and rude who seriously swear in Japan. Most upstanding Japanese people consider such language grossly impolite. They tend to use euphemisms (enkyoku na kotoba) and adore circumlocutions: The rule of thumb is, "Don't call it what it is."