Agreed. Personally, I don't regard it as an "illness" at all. It's simply a perversion, on par with beastiality, S&M, pedophila, and numerous other expressions of sexual deviancy.
Do the varied personalities of human beings lend themselves to such perversions? Certainly they do. A person who is especially drawn to children can be lured into desiring sexual intimacy with them if he (or she) does not resist the temptation. So, too, can a person with a greater affinity for the same sex, or whose personality lends itself more to manifesting the traits of the opposite sex, be lured into desiring to a sex change, or at the very least into playing the part of the opposite gender during sex.
Does this mean such urges are an "illness"? No. Even husbands and wives can sometimes "switch roles" during sex because of the dynamics of their personalities and their relationship together. It simply means urges to practice sexually deviant acts outside what God has commanded are a sexual lust and a kink, and must therefore be resisted.
Granted, the atheist and the unbeliever have no reason to resist such urges, since they have no fear of God and no faith in His existence.
Time will tell if there was reason to fear Him or not.