But let's say I proposed it as a canidate for a mental disorder; I understand we are not assuming it to be as such by default. What criteria would have to be fulfilled to be considered a mental disorder?
Well, the following resolution was produced by the APA:
Homosexuality per se implies no impairment in judgement, stability, reliability, or general social and vocational capabilities
The only homosexuals that were investigated before this, were insane homosexuals so the fallacious link was made that homosexuality is a mental illness. Your criteria then are that you need to be able to demonstrate an impairment in judgement/stability/reliability/social skills/vocational skills. In short - you have to show that they are impaired in some mental fashion.