I'll try and all, but I just have to say that it'll be difficult for me to objectively grade a paper arguing that homosexuality should be a mental disorder.

It's either a mental disorder, an emotional disorder or a physical disorder. Take your pick.

Or, if none of that pleases you, provide some sort of argument that demonstrates that homosexuality is not a "disorder."

Before you make an attempt at that, let's establish a functional definition of "disorder." A common sense approach might dictate that we define disorder as "abnormal and maladaptive behavior" (though I would prefer to drop the "abnormal" descriptor and just stick with maladaptive behavior). For simplicity's sake, abnormal can be taken to mean statistically abnormal. Maladaptive behavior can be taken to mean behavior that is generally counter-productive to attaining happiness, enlightenment and self-actualization.

Currently, in today's heterosexual culture, homosexuality is both statistically abnormal and a barrier to the attainment of happiness.

Any sort of behavior that results in censure by the majority is probably maladaptive. If you're inclined to argue about whether homosexuality is a barrier to the attainment of happiness, ask Matt. He was maladapted with the butt end of a pistol while tied to a fence. You can't claim that this was healthy behavior for him to engage in.

Damn gays.

Perhaps in another few decades, we'll reach a point where homosexuals are no longer persecuted for their behaviors. In the meantime, homosexuality will continue to qualify as a "disorder."

Speaking of infrequently discussed disorders, you may have heard about (or perhaps even met) certain unfortunate individuals with a maladaptive skin disorder that results in an abnormal abundance of melanin in the skin. One wouldn't necessarily think that this would result in any significant difficulty, but for some reason it seems to cause the same sort of problems for its victims as homosexuality.