Theon loses a bet that means, for the first time, Jon gets to fuck him, rather than the other way around. He assumes Jon will treat him as roughly as he's always treated Jon. This proves incorrect.

When Jon realises where Theon's looking though, he turns bright red and hurriedly covers himself with his hands. “What are you staring at?!” Theon forces his jaw to close again. Great, now the bastard's going to think I'm some sort of deviant. Well it's his bloody fault; if he's going to show off that monster he should expect the odd glance. “Nothing Snow, just figuring something out,” he says, forcing his usual mocking tone, “I know your heritage is a bit of a mystery, but I think you might be part-centaur.” At Jon's bewildered look, he rolls his eyes. “Because you've got a horse's cock between your legs.”