Well, it was a valiant effort on Kat's part to say the least.
The money line aside, I have to say that I see Kat's point. So long as it isn't pornographic, something silly-sexy and yet Christmas-themed would indeed be very, very Art and something that grandma would hopefully see as nothing more shocking than a classic seaside postcard.
Coincidentally, Kat has also told Art what he needs to do for her Christmas card!