Just how do people get into these weird fetishes in the first place?

My theory is that it’s just moving the goalposts. People start getting off to one “normal” thing, but in their pursuit of self-pleasure you move ever so slightly into more obscure subjects. Your mind begins associating certain themes or objects or situations with sexual gratification, and those things act as an anchor for the subsequent times you get off. Your mind relates two unrelated things the same way it does for irrational fears or memory shortcuts. The brain rewires itself all the time, so it’s very likely that those associations eventually turn into fetishes