honeymoon joke (Washington Sq. Park)

After they told this joke one of the women turned to me and said, "You get it?" Now there's only one answer to this question, which we all learn in second grade when we get asked to spell "I cup." I answered the question properly, but I must admit here, before the world, before the grown up kid who asked me to spell "I cup," I did not answer truthfully. An explanatory email thrown my way would help.