I have yet to lay eyes on a bum
without thoughts that include the word yum.
Though I hasten to add
it's a gender-linked fad
bony boyscouts will render me numb.

We Down Under say bum and not butt,
you drink rum, that is open and shut.
We don't ask you for much
but we do like to touch
woman's butt beats the tastiest nut.

Herbert Nehrlich

http://www.poemhunter.com/poem/limerick-games/