okay it goes roughly like this (altered to make it more fanpop compatible)
guy to girl: whats the difference between peanut butter and jam?
i can't peanutbutter my privates up your erm other type of privates
...now either google and read the real version or think of cruder words for male privates..and think along the lines of back entrance for the girl.
this also comes in different versions of shoving parts down throats etc..be creative! :P

You can google it. The answer is out there and easy enough to find, but I don't think it's appropriate to put on here. haha

I heard a version of this joke years ago. The cleanest I can write it here is: "Jam doesn't look like poo when u eat it out of a girl's ***hole"

Either way, I love the irony of the man with one of the filthiest mouths in comedy saying that this is the dirtiest joke he ever heard. I wonder what Ted would think of Bob saget's version of "The Aristocrats" :P