I must admit that I'm intrigued by the idea of "Playing Lyrics" - I assume you mean accompanying songs with lyric which you deem "filthy". I myself wouldn't describe anything of a sexual nature as "filthy" - I could imagining using words like "Risque", "Bawdy", "Crude", "Disgusting", "Perverted" etc. Most of the lyrics which have been mentioned would get get no further down the list than "Bawdy" although some (mainly due to their lack of sublety) might just creep into the "Crude" category.

In the UK we tend to call the more extreme examples of this genre "Rugby Songs" - named after the game (or rather the boozy post-match sessions) rather than the town. Many of these lyrics have been compiled with some measure of skill - even if it's only the ingenuity with which rhymes are found for many bodily parts and sexual functions. Some are quite clever (The sexual Life of the Camel for instance).

There are also many other songs - a large number of which can be found in the DT under the category of "Bawdy". Many of these have their roots in the Music Hall, others come from the armed services and a large number of songs (mainly employing inuendo, phallic symbolism etc) come actually from the folk tradition itself. For an example from Tudor times see Watkins Ale.

If you are performing in venues where the clientelle request such songs either sing them, or if it upsets you, find other more genteel audiences to entertain. I do sing the odd bawdy song ("The Lobster Song", "Gentlemen will please Refrain", "Gladiator Song" etc) and find that in some circumstances that they can lighten the atmosphere and shut a noisy audience up. "The Thrashing Machine" did exactly that at Tenterden FF last Saturday - Fellow Mudcatter "Synbyn" complimented me on getting the audience "back in listening mode". The important thing is to use these songs wisely - ration them and find other songs with interesting (non-bawdy) words to sing when you've got their attention.

Incidently, I've earned many a pint though a contest to see who knows the most verses to "In Mobile".