[Polycarp]

then how do you separate out which part of the behavior is caused by the mental illness, and which part is caused by the sinful nature?

That is left to the Mercy (and Justice) of God.

None of us can judge the culpability of anyone who commits what is, objectively at least, a mortal sin.

By the way, no one has "debunked" reparative therapy, and neither can they, because it has proven to be effective for at least a percentage of all homosexuals.