I am not sure how much this will be in line with what you want, but my suggestion would be to use the existing arousal and fetish system in place already. You would need another variable for state of a specific fetish, which should be neutral at the beginning. Any time an act causes arousal it would increase the specific fetish stat (experience). This is already implemented if I understand it right.
I think it is nice to give the player some control over how Sveta develops though, so once a fetish exp stat hits a certain value (which may or may not be different for different fetishes) Sveta gets a choice to make, for example in the form of a dream (or at least at the end of the night), with the options to be
-completely disgusted by the fetish in question, in which case the fetish experience will be ignored subsequently (as if it was 0); alternatively it could count as "degree of hatred of a specific action" if Sveta got exposed to it a lot after she decided she hates it
-reluctantly accepting it, in which case the fetish experience will count as "desensitizing" in the way a previous poster said
-embracing it, in which case the fetish experience stat will now also count as fetish preferrance (maybe with a modifier)
This choice will then determine what the specific fetish state variable changes to. Any writer writing new scenes or improving old ones can then use the specific fetish experience or preferrance (both the state variable and the exp variable) relevant for the scene to determine how it would play out. If a general corruption variable would still be needed it could just be the sum of all the experience for the different fetishes that Sveta is not disgusted with. (Or if the writer thinks that even a completely kinky Sveta would still be abhorred by the act (or the other way round) they can just ignore the fetish variables for that scene). A specific scene might even subtract fetish values to get to a result (in the case the "degree of hatred" would get implemented - A Sveta with a sub fetish but a maso hatred could use this to determine if a scene with both would be a good or bad experience for her).
After thinking about it a bit more I came up with two potential problems:
a) conflicting fetishes, if an 'arousal' call would contain a fetish that Sveta likes and one that she hates - after more thinking on it I decided that this would require no action, as Sveta could still develop "fetish1" more even if she hates that it is done at the same time as "fetish2"
b) reading through the code I was wondering why they were so many 'arousal' calls including both 'bj' and 'sub'. I am still not sure how to feel about it - in my opinion a random bj would not necessarily increase submissiveness, on the other hand I can see why a submissive fetish would increase arousal during giving a bj. Because of that I am not sure if sub['exp'] should really measure Sveta's submissiveness - this game has the option for a lot of random bjs, which would increase that stat very quickly.
Finally I would like to add that aside from those fetishes the game already lists Sveta could also develop a preferrence for certain sexual acts such as blowjob or even kissing - there could be an option of treating these in a similar way as the fetishes (or at least preferrances).
