"why are people suddenly saying kink isn't always sexual" hello! you seem to have picked up on something that is actually de facto considered true in IRL kink scenes and often trotted out as one of the basic foundational truths of kink in educational contexts (i.e. beginners' workshops, it'll be in guides for first-timers at dungeons, etc). come take my hand and let me introduce you to the vast and wonderful world of IRL kink.
"kink," contrary to what a lot of internet-only kinksters seem to think, is not referencing your relationship to a concept--it's referencing the concept itself. for example, "bondage" in this context is not referring to "being turned on by tying someone up/being tied up," it's referring to the concept of being tied up/tying someone up itself. this is one of the most common kinks you'll see people playing with in entirely nonsexual contexts--most rope groups have at least a handful of players who are asexual or entirely sex-repulsed, for example. at a lot of rope play sessions, sexual activity will be banned entirely & you'll generally be expected to not show signs of sexual arousal while in that space. because it's not the space for that--sexual play parties are their own thing.
so what makes something a kink vs. a "normal" interest, if it's not a sexual aspect? mostly social stigma. the group of interests considered "kinks" have developed a shared umbrella due to a shared societal pathologization of those interests in similar ways, and one of the ways that pathology manifests is by considering those interests fundamentally sexual. there is absolutely nothing wrong with having a sexual dimension to your kink, to be clear--but society sees sex as inherently disgusting, and sexualization is frequently weaponized to pathologize all kinds of people.
portraying interest in domination/submission as inherently sexual, for example, means those explicitly navigating consensual d/s play are relegated to the realm of "the bedroom" and signifiers of their relationship (i.e. a collar) are considered "obscene" and not fit to exist in public--even when equivalent signifiers of vanilla lifestyles are allowed and not considered obscene, i.e. a wedding ring. if you portray an interest in roleplaying as a dog as inherently sexual, it means anyone roleplaying as a dog in public is subjecting the public to nasty, perverted sex, it can be classified as obscene, you can arrest people for barking and wearing leashes even if they are deriving zero sexual pleasure from it whatsoever and are not exposing themselves or harassing anyone.
why are most kinks expressed sexually to some degree, or have a sexual dimension to them? because sexually active adults tend to enjoy sex. i don't know how else to explain this, genuinely. if you are a person who experiences sexual arousal and sexual attraction, then you will often be aroused by your interests in a sexual context.
like, i am really into fandom. i love a lot of different things about fandom. one of the things i love about fandom is that it's horny; i'm a big enthusiast of smut, horny cosplay, et cetera and so on. but that doesn't mean that i don't like fandom in completely non-horny contexts, or that my interest in fandom is always sexual and can never be separated from sex. just because i cosplay erotically doesn't mean that i don't also cosplay in other contexts entirely. cosplay turns me on, and i also love cosplaying at conventions and for entirely non-horny reasons.
kink is the same. sometimes the exact same person is both turned on by petplay and interested in it nonsexually. this is actually like... really common, it's not even just "sometimes," i would say it applies to the vast majority of IRL kinksters active in my local scene, and i would hazard a guess that this isn't a major outlier, it's fairly common in scenes across the world. but if you can understand "i like cosplaying, and sometimes i'll cosplay at a convention, and sometimes i'll cosplay for sex, and sometimes i'll cosplay for fun alone in my room, and sometimes i'll cosplay to jack off, and sometimes i'll cosplay for money, and none of these are The Only Reason I Like Cosplay, and sometimes when i cosplay i will do it in a way that only includes some of these reasons and not the others at all," it shouldn't be too hard to apply the same reasoning to kink.
anyway, trying to relegate "kink" to only and solely refer to "this set of stigmatized interests and relationship dynamics, but only in reference to sex specifically" is closing the barn door after the horses are gone. sorry y'all but the kink community developed in response to medical pathologization and severe social stigma. there's no post-hoc way to turn it into an ontological classification of some form--kink is messy and abstract as a category because it isn't a category based in, like, scientific observation or ontological meanings of words. it's a category based in "we think these people are freaks and we need a diagnosis for their freakishness" lol. the forcible sexualization of every aspect of kink is part of the initial classification of this loose social realm as "fetish" and/or "paraphilia." sex is not evil or scary or bad but equally the structures doing the pathologization do consider sex to be evil and that's not something you can just ignore when discussing how sex is utilized rhetorically in these discussions.
