Infidelity joke

I read it in Calvin and Hobbes:
Girl: Oh dear Simon, your company is such a pleasure. *puch* *puch* (kissing sound)
Boy: Indeed. *smooooooch* (a long kiss)
Girl: Wouldn't it have been great had we been married? *puch* *puch* *puch*
Boy: But we married... *smooch* *smooch*
Boy: or do you mean, married to each other