It seems useful to have some plausible definition of mental pathology - psychological pathology or psychopathology - to set against the pernicious inversions of mainstream culture.
Mental pathology is associated with a probable reduction in (proximately) survival and (ultimately) reproduction.
Reproduction is the key; because it is this which links medicine to biology, to the modern (post 1950s) understanding of biology as the science of entities that have been subject to natural selection.
So, in this objective sense, a mental pathology is not about what makes someone happier or more miserable; but about the effect of consequent behaviour on probable reproductive success. Does the phenomenon make that person more, or less, likely to conceive and successfully raise children?
This definition is particularly useful in clarifying the situation with human sexuality - where people get endlessly confused trying to predict what conceptualisation may optimise human happiness (in individuals or among groups); when the biological reality is crystal clear. 'Abnormal' is pathological when it reduces reproductive success.
The flip side is that a biologically valid treatment should restore or enhance probable reproductive success... Most obviouly by extending expected lifespan, but more fundamentally by making it more likely that a person will have children, and raise many biologically-viable children.
Interventions that - by contrast - reduce or obliterate reproduction (such as 'gender reassignment') are revealed as Not being treatments.
A softer, but useful, aspect of mental pathology is that it does not come as single isolated traits, but as clusters of several or many reproduction-damaging features.
There may well be some undiscovered unitary underlying 'lesion' that causes this variety of observable pathological symptoms and signs - such as a gene mutation, or a structural brain abnormality, or a chemical change somewhere - but such unitary causes nearly-always cause multiple adverse consequences; because organisms are made of linked and interdependent systems.
So, again with human sexuality as an example; when a sexual phenomenon is associated with increased rates of other pathologies e.g. other associated mental illnesses, increased rates of self-harm and suicide, increased annual death rates... then that behaviour is plausibly pathological, by normal medical and biological standards of evaluation.
Of course, modern Leftism operates by exclusion of normal medical/ biological criteria, by inducing and maintaining permanent confusion, and by imposing pseudo-solutions by massive propaganda and overwhelming coercive force.
Nonetheless, in this spiritual war we can know what's-really-what with mental disease, if we bear the above criteria in mind.