I did some extra research, and yeah, turns out something being a mental disorder does not depend on whether it involves atypical behaviors or preferences, it depends on whether it causes distress or impairment, or is harmful to the individual or others. Furthermore, the context, intensity, and impact of the behavior is also a deciding factor.
Basically, it's subjective. It's a disorder if we decide it's a disorder.

[–] [email protected] 5 points 11 months ago

I would not go as far as saying it's subjective, there are official definitions like the who's :

https://www.who.int/news-room/fact-sheets/detail/mental-disorders

But yes it depends on the context and the period so it's not entirely objective either.