The "Frank is a control freak" theory stems from his ex-wife calling him out over
sabotaging her writing career. Frank tried to play victim and claim she was the abusive one in their marriage, but by her recounting, he forced her to stop writing, destroyed her self-confidence, and when she tried to get away, stalked her across the country and got into her apartment without her knowledge. Y'know, like normal people do.
Is it possible to confirm any of this? Hard to say, and I'm certain she'd rather not discuss the matter of an abusive relationship further. I'd certainly trust her account over Frank's, anyway. But we do know that Frank's a pretty fucked up character in general; he did marry John Walker Flynt, after all. The armchair analysis is that Frank doesn't like when things aren't in his control (if you really want to go armchair psychiatrist, maybe it stems from being unable to save his sister from drowning), so he set up a relationship where he holds the reins alone. John would be absolutely helpless without Frank's money; he has zero skills or social abilities, and his notoriety would only go so far in landing him a job (that he'd probably lose). Frank knows better than anyone that he wears the pants in the relationship, and that there will never be any risk of John becoming independent.
Thus, Frank doesn't really care that John makes fun of him on Twitter constantly, portraying him as an ignorant buffoon that can't compare to his super awesome hacker biker gamer politician celebrity wife. He's probably happy to go along with it since it gives him perfect camouflage. Nobody would suspect goofy, affable Frank Wu of being a monster, after all. And if he ever gets tired of being married to a wendigo, he can easily claim an abusive relationship, pointing to hundreds of tweets of belligerent behavior towards him, along with just making up a bunch of shit that nobody will bother to check into. John's a 6'+ ape, I think it wouldn't be hard to fake some injuries and claim he did them.
But I doubt that would happen. Frank has his perfect relationship, and even though he's married to a complete parasite, he has complete control over John, and that's all that matters.