So one of Bill's fantasies of Dipper is that he's a submissive, pliant and even more lustful husband who worships the grounds Bill walks on complete with a revealing slave outfit? Wouldn't that fantasy contradict with his real enjoyment of the rebellious, sarcastic and equally capable Dipper in real life?
This is one of those things that’s fun for Bill to fantasize about. He wouldn’t want that all the time, or he’d get bored of Dipper fast. But the idea of Dipper doing it has a thrill to it, because it’d be exotic coming from his feisty little mortal.
Dipper actually doing any of that would quickly kill any fun about Bill’s fantasy, if only because Bill would be out to find out what screwed up his nice defiant human.