"For this dance, decide which one of you is going to be the lady and which one of you will be the gent.  Put the lady on the right. Don't worry, it's not permanent, I promise.  In another dance I'm gonna call you "foxes" and "squirrels" - they're just traditional calls that rhyme well - I promise no one will actually sprout a tail or any other bodily appendage."   or "Decide who's going to be the lady & who's going to be the gent.  Don't worry - I *know* none of you are really [gentlemen/ladies] - That implies good manners and decorum." or "Decide who's going to dance the role of Lady and who's going to pretend to be a Gent.  Someone told me once I should use other non-gender-specific words like "Aardvark" and "Duck", but I kept getting into trouble with the rhymes.... So now I stick with Lady and Gent.  Much safer."