I've mentioned this before, but I personally theorize that Frank simply gets off on the fact that he has absolute control over Brianna's life in every way possible. When he first met her, she was a nearly 30 year old John Flynt living with his aunt and going nowhere with his life, and after they got together John got his botched ball-chopping surgery, got married, and moved in with Frank and she's never had to work another day in her life so she's free to spend as much time and money as she needs to play pretend. John always likes to paint their situation as a little slice of heaven on earth, this is simply because he's such a narrow-minded Narcissist that he believes as long as he's provided luxuries and is free to REEEE into ether while living in his own fantasy work their relationship is happy and healthy. The truth is Frank is a known controlling abuser, and to someone like him Brianna is a godsend. Without Frank's name and money Brianna would be nothing, and she's so stupid she won't question anything about their relationship so long as she gets to maintain the image she wants to present. She's too lazy to ever try to leave him because that would mean she'd actually have to start working and wouldn't be able to treat life like a game anymore, she's too ignorant and naive to ever realize the kind of situation she's in, and she's has absolutely no useful skills that would ever allow her to rise above him and damage his ego. Really Brianna is more of a hobby than a spouse to Frank. Even if he has to devote a bit of time and money on asinine projects to keep her happy, he's still got somebody that has no other option in life than to be with him, and that's all he needs.