Seems to me from some of her 'dreams' and 'flashbacks' or 'recollections' that she was always a little bit of an exhibitionist, just it usually only came out when she was drunk and Luke found he liked to watch other guys play with Katheryn when he caught his 'friend' playing with her tits that one night. Once you find this stuff out about yourself it doesn't usually take that long to get in to it especially if given encouragement by someone you love and trust. Usually the only thing that would hold you back would be your own insecurities.
Luke is a control freak, he hates the idea of Kathryn making her own decisions and he's done everything possible to make sure she always checks everything with him first, before making any lifechoice. So it's not surprising that Kathryn is hesitant and unsure about what she wants, and always feels that Luke has to be kept in the loop. My guess is, one of the main reasons he chose Kathryn is because she was insecure, and thus easy for him to control.
And he's the kind of guy she would naturally go for, because she believes she needs someone else to make her decisions for her. And Luke is also not nasty about wanting to be in control, like Mr Santini, at least not yet anyway ( that's why Santini says him and Luke are the same, he recognises a fellow control freak). It's noticeable, that everyone says that Luke's a really lucky guy to be with someone like Kathryn, as if they believe she's really out of his league. The truth is, she is out of his league ( I think that's what he believes anyway), but by making her so dependant on him, she never considers any of the other possibilities that could be available to her.
Luke's problem, is that his 'fetish' is now putting all this at risk. Because by pushing her to be bolder and take more risks sexually, he's unintentionally making her bolder in other ways as well. She's starting to consider a very dangerous possibility. She really doesn't need Luke to make decisions for her. This is why we start to get choices for Kathryn alone. It's clear evidence that she's starting to think for herself, without having to clear everything with Luke first. This is Luke's greatest fear. Because if she starts making her own decisions, what does she need him for? She might start believing she really is out of his league, that she could do a lot better someplace else. The control freak in him, simply cannot conceive that she might like him for other reasons. Because the hard reality is, that it's Luke himself, not Kathryn, that believes she's out of his league. That's his insecurity, which he hides from the world, by being a control freak. His cuck fetish, is born of how he really feels inside, helpless, out of control, unable to influence events.
I'm not trying to suggest this is all some devious plot by Luke to manipulate his girlfriend and deliberately keep her under his thumb. I'm pretty certain, it's all unconscious on his part and he truly believes he's doing all this for her benefit. But the reality is, that it's mainly for his benefit. Kathryn is quite capable of making her own decisions, and in any normal relationship she would be encouraged to do so. We don't learn from always making the right choices, we learn from sometimes making mistakes and then not making them the next time. That's how we transition from child to adult. Luke has decided he needs to make sure Kathryn never does this, so there's no possibility she'll ever leave him.
But giving in to his fetish, is sabotaging this, because it's forcing Kathryn to become more adventurous, and the more adventurous she becomes, the more likely she is to think for herself. So it starts off being purely for his pleasure, but she enjoys it. And gradually enjoys it more and more, until it's not just for his pleasure but her own as well. How long before it's mainly for her pleasure, that he doesn't need to be a part of? And we see this happening, when she teases Alex without checking with Luke first. She says to herself, that she'll tell him about it later, but she doesn't. She fantasises about Franklin and keeps that to herself as well. She ignores his warning about Santini and takes the job, because she decides for herself that having enough money, is more important than Luke's misgivings about his boss. On the surface at least, it appears that Luke has more control over her, but underneath he's actually losing it, because she's becoming more and more self- confident. And confident people, don't need others to make decisions for them.
Kathryn's mother was a model, and it's obvious she's an exhibitionist, who likes to take risks. So not so surprising perhaps that her equally beautiful daughter takes after her, even if she's tried really hard to deny it to herself. Kathryn is a natural model, but until she tries it for herself, is convinced that it's not her at all. Other people can see it, but she is blind to it. She obviously lacks confidence in herself, and Luke has cultivated this belief, so he can control her. Deep down he knows, she could easily be a model, and she could do pretty much anything she set her mind to. But he doesn't want her to think that way, because she might decide he's not good enough for her. So he suggests she might be good enough to be a model, but never really encourages it, subtly insinuating the belief in her mind, that it would be just too much for her. So on the one hand, he's always surrepticiously putting her down, but now his fetish is having the unintended consequence of building her up.
The supreme irony is, that the more he seemingly controls her, by persuading her to do more and more sexual things, the more independant he's actually making her. And he's setting in motion, the very thing he fears most. She may come to believe herself, that she's out of his league? Now I'm guessing, that will depend on your choices as a player. If he cheats on her, she'll probably find out about it, and end up cheating on him, and their relationship will fall apart. But it doesn't necessarily have to end that way.