The author's poorly disguised fetish
The author's proudly displayed fetish
The author's fetish you're pretty sure they don't realise they have
The author's fetish which they're firmly convinced everyone has and is just pretending otherwise
The author's non-sexual special interest which just sounds like a fetish because of their habitually unfortunate phrasing
The fetish the author is making a well-meaning effort to cater to in spite of clearly not understanding it themselves
The author's fetish that never quite makes it into the text because they keep getting sidetracked by the requisite worldbuilding
The author's utterly pedestrian sexual preference which the text treats like a bizarre fetish because they've got shit to work through
The author's seemingly innocuous recurring trope they're going to have a personal revelation about ten years down the road
The author's fetish you missed on a first reading because it's so far out of pocket, it never occurred to you that you could sexualise that