Check out
last year's winners. (Brendan finished second). You'll notice that the winning limerick did allude somewhat to sex, but isn't "dirty," and most of the rest are clean as can be. Before you say clean can't be funny in a limerick, consider the #3 winner by Stephen Gold as a refutation:
A mathematician named Fry
Was the shape of a sphere. When asked why,
He replied, “That’s abstruse,
But I roundly
educe
My circumference follows from pie.”
And Ann Martin's #4 limerick is also clean and very clever;
Anatomical study will show
That five letters are all you need know:
The
ELBOW is placed
Somewhere over the waist,
While the BOWEL is found down BELOW.
Both Stephen and Ann are UK denizens, by the way.
PS--
You don't just have the weekend. You have all day Monday as well, right until midnight (and, I suppose, a few extra hours if you're in UK time).