Well that's just your opinion. Some of us have a different view. The whole point of corruption for a lot of us, is that the protagonist ends up getting corrupted, and starts enjoying an entirely different sexual experience, that deviates utterly from the norm, that you want her to fight to preserve. This is a fantasy game, where there's no real downside to ending up in an unconventional lifestyle. I assume it's uninteresting to you, because you don't want that, but it's very interesting to me, because I do. Not because I particularly want my life to be like that, but because I want the protagonist living in her fantasy world, to enjoy something that wouldn't be very practical for me.
Yes, my whole joy in playing these games, is getting the protagonist to succumb and be transformed from a loyal if frustrated wife or girlfriend, into a complete slut. But even when she starts cheating, I have no idea if she'll eventually reach that point. She might only go some way to becoming a slut, before the Dev either gets cold feet or he never intended for her to go that far anyway. I never really know if my particular itch is going to be scratched, and that's what keeps it interesting. And people who have different hopes for the protagonist's eventual corruption are in the same boat, and feel the same way I'm sure. We're all hoping for the outcome we want, and if the Devs good at his job, he'll keep making as many people as possible, believe that they'll get what they want....eventually.