[1.] The lim'rick packs laughs anatomical
In space that is quite economical,
But the good ones I've seen
So seldom are clean,
And the clean ones so seldom are comical.
[2.] Legman, who compiled the largest and most scholarly anthology, held that the true limerick as a folk form is always obscene, and cites similar opinions by Arnold Bennett and George Bernard Shaw, describing the clean limerick as a periodic fad and object of magazine contests, rarely rising above mediocrity.
That said, I don't think an excess of filth, whether or not it's folklorically accurate, is particularly funny either. One, or, perhaps I should say 'the MC community' needs mostly clean stuff to throw the filth into sharper relief. So I think my conclusion is to bring on the odd willy joke, and not to start complaining until we've had several in succession.