It has been proven by history. Homosexuality once was classified as a mental disorder. It was finally disqualified as such entering the DSM-IV as it is considered to be a sexual choice genetically. If homosexuality is (still) considered to be a disorder people would definitely be harsh on it, as it implies that it is a disease and with every illness a cure is expected. Most importantly, classifying it as a disorder would label the act/will itself, and we shouldnt.
I mean, what the government says isnt like a revelation from God or anything! So yeah, there will still be reasonable people, or true believers of Christ, that will still know, no it is NOT a mental disorder, its their direct choice to commit this horrible act of sin so therefore they are not excused. Honestly Christians who say its a mental disorder as a way of saying its bad, why? If it was truly a disorder, then they truly wouldnt have much control over it but all sin is an active choice. There is no excuse for sin.
People were more harsh. This calls the whole idea of "disorder" into question. Some disorders are definitely real. For instance schizophrenia involves not knowing what is real and what is not. But then how much of "mental disorder" is defined as such because of subjective preferences society enforces about how people should think and behave?
An odd thing is when it comes to homosexuality people who say it's a mental illness often denounce findings correlating genetics or brain chemistry or hormones with homosexuality. Yet for anything else when trying to argue that it's a disorder those things are used as supporting evidence.
If it doesn't involve a break with reality or loss of control (by loss of control I don't mean others perceive the person not to be in control, I mean the person quite literally wants to do one thing but is compelled to do another i.e. OCD) then mental disorder is based only on entirely subjective standards for how people should behave. This is not to say society should not regulate behavior, but labeling anything we don't like as "disorder" only sends a confusing message and encourages people to identify internally as "ill" rather than as "misbehaving" and hence is bad for getting people to take responsibility for their actions.