A bit off-topic, but in my case, I always thought feet were ugly. Then, one of my first sexual experiences as a teen happened to involve feet. Over time, I noticed I was slowly developing an attraction to them (first only pantyhose, then barefeet too, nowadays even socks and their smell). I honestly believe you can rewire your brain to develop fetishes. If something gets repeatedly paired with sexual pleasure, your brain starts to associate the two, even if you didn’t find it appealing before.
Basically classical conditioning.
Can relate to that 'experience during teens' part.
There's also the part about : This part of the body isn't supposed to be associated to sex. And doing make it feel like doing something forbidden (even if the word is a bit strong).
Also the link to sub/dom play, with feet being the lowest part of the body, something used to walk (touching the ground), 'dirtiest' part, etc. And thus taking pleasure from that part of the body create that sub-dom / humiliation feeling.
And there's also, for me, the fantasy/virtual part. Many of the fetishes I enjoy in games & doujins, I do because they are in games while I would most likely be rebuked and/or disappointed irl.
The screen protection and the awareness that it's not real which allow this fantasy about how pleasurable it appears to remain as such. Mostly enjoying and taking pleasure in seeing the characters (including my character that I control, if rpg) taking pleasure, often times amplified pleasure, in those act.
If what I wrote makes any sense, feel like I burned my brain trying to translate feelings to words...
Edit : Btw, Bonds Defiled is an awesome game, one of my favorite find of this year. One I frequently go check on to see it's progress. Keep doing what you're doing !