NSFW tag because I can’t really talk about this without mentioning how it’s often weaponized against furries/other kink communities - one example in particular that comes to mind is how radfems will claim that anyone who is a BDSM sub needs therapy. It’s a way to dress up condemnation of others in virtuous-sounding language - “I’m just concerned about their mental health!” - and carries a lot of ugly implications:

• Scientism: A lot of people making these claims take up the language of psychology without looking at what actual psychologists have to say. I’ve seen many behaviors/kinks (again: radfems and BDSM) get labeled as “insane” by people who have a layperson’s understanding of psychology at best and who completely contradict modern psychology at worst. All they really do is give a false sense of authority to their own prejudices.
• Ableism: These accusations often get leveled against neurodiverse people by a society that, while having seen some improvements, largely sees autism and other manifestations of neurodiversity as something “wrong” that needs to be forcibly corrected.
• Infantilization: Implicit in accusing someone of being insane/unhealthy is the idea that they are not a full adult person - that is, they can’t be trusted to make their own decisions and need to be controlled by someone more “competent.” This has historically been used against a wide variety of marginalized groups to justify depriving them of their rights and self-determination.
• Violent Implications: Modern society’s treatment of those it deemed “insane” has historically been horrific, locking them up and imposing upon them damaging and torturous “cures” such as lobotomization, conversion therapy, ABA, and basically anything done by the troubled teen industry. Even in institutions that are above-board and operating on the latest and best knowledge of psychology, neglect and abuse of patients is distressingly common. Implicit in the accusation that someone is “mentally unwell” is, at best, to say that it’s justified to force them into a situation where they are at a high level of risk for abuse and neglect.

Hopefully this goes without saying, but obviously this does not apply to kinks/paraphilias/other non-normative behaviors that actually, provably harm others. If someone’s actually hurting people (or is a credible danger of doing so), they need to be stopped. Pathologizing a behavior carries its own inherent risk of hurting innocent people, though, and for that reason it demands a high standard of proof.