Mara spent a lot of time during the trek on Myrkr looking at Luke's butt in his tight pants and pretending she wasn't. Years later he tells her he knew all along - he's used to people looking at his butt. And taking sneaky holos of his butt. There was even a competition the Rogues ran on Hoth to see who could get the best butt shot. Han won. Mara goes straight over to his and Leia'x place to see if he still has the holo. Of course he does, and Mara adds it to her collection.