As far as we can tell, Flynt has been this batshit crazy since at least the Ole Miss days, pre-trans status.
At some point, Frank Wu thought Brianna was worth marrying despite being a raving, psychopathic tranny who had nothing but a trail of failure called Socially Unconcious behind her.
I suspect it has been a marriage of what was initially convinience (because of Flynt's money at the time, though God only knows why Brianna married Frank - might be a dominance thing since in the wild it was observed Bri treats Frank like a stereotypical evil villain treats the underappreciated henchman who literally does everything for said villain) and now they're inexorably tied to each other for at least one strong reason that, should they divorce, would bring about ruin for one or both of them.