As for the topic, I don't understand why they can't just go by the general rule of "if it is between consenting adults and nobody is hurt (mentally or physically) in the near or long term then it is probably okay" rule. It seems like it would make things much more easier.

Well, as far as I can see, that is not so easy, since the "nobody gets hurt (mentally or physically)" thing can be very tricky. Remember there are millions of possible cases out there.

First of, I have no real knowledge of the subject and am talking right out of my posterior here. But since few here seem really qualified to talk on the subject on a scientific level, that's the way to get a discusion going I guess.

I have heard (having no real knowledge on the subject) that a rule of thumb of what is a mental disorder and what is not is defined by the question: Does it interfere massively with your life? Of course there are mental illnesses that are really neural illnesses and can be detected and defined otherwise, but the question if you are e.g. depressed isn't defined by a hormone level of this or that, but if your depressive symptoms are graet enough to sufficiently interfere with your life. Same goes with most other mental illnesses AFAIK: As long as they don't really interfere with life, they are not an illness, only when they become obstacles.

Now, speaking of sexual things, that can be a very difficult question. Love and sex are famous for making everything complicated, and the most vanilla sexual desires have fucked up many a life.
Example: Quite some republican's political career has been destroyed because of homosexual longings the person had, but did not want to have, thus seeing homosexuality as something evil, campaigning against gays, then giving in to his supprressed longings, being found out etc. We all know the story. If you ask said republican, he would probablY say (of course not in public) that homosexuality destroyed his career. So, people get hurt because of homosexuality? Is it a mental disorder then?
It is hard to draw the line where the desire itself is the problem or the reaction to the desire or something else that just looks like the effect of the desire.

Since the manual doesn't even go as far as "disorder", but talks about "deviance", it becomes even more complicated. Wikipedia defines "deviance" as actions or behaviors that violate cultural norms including formally-enacted rules as well as informal violations of social norms (e.g.nose-picking). Sometimes I feel that both having sex itself is breaking a social norm (seeing how it is still tabooed in many ways) as well as having no sex (seeing how media present it). So I have no idea where a behaviour that probably happens in a private context violates informal social norms.
On the other hand, I agree that it is not nice to be labeled a deviant because of what one does with one's grown-up partner at home.

Hmmmm. The problem may be that the term deviance isn't one that just makes sociologist express a happy "what a fine specimen" when they meet someone labeled thus, it makes non-sociologist go "what a freak". Perhaps the term had better be changed into something neutral? On the other hand, gay seemed to be a word for happy originally, and that didn't help. But I have certain doubts that "breaks a social norm" can really be defined in an exact scientific way, seeing how diffuse those norms are. So maybe an in/out dichotomy like deviant/not-deviant is just not applicable, and should be forgone for an "common/uncommon" dichotomy that is more neutral.

Dan Savage once wrote in one of his columns, being gay was once labeled by a scientist as "abnorm", and when people protested, the scientist said it was a neutral term, meaning it was not the norm. If said scientist was not disingenious but simply naive, it would have been somewhat similar to this: The scientific use and meaning of words can be very different from the general meaning, and when both are mixed, things often go boom.  

So in short, I am happy I don't have to sit on that defining panel: Whatever they do, it will probably be wrong.