Scientific evidence has confirmed that genetic and hormonal factors do not seem to play a determining role in homosexuality (Birke 1981, Perloff 1965, West 1977). However there continue to be attempts to prove that genetics rather than family factors determines homosexuality. These continuing efforts reflect the persistence of gay advocates to formulate a means by which homosexual behavior may be viewed as normal.
in Reparative Therapy of Male Homosexuality
I don't think it's entirely fair to criticize a 10 year old book in terms of today's scientific knowledge, but it certainly underlines why one should cautiously if ever claim that scientific evidence has "confirmed" anything. That's not really how science works. Depending on the skill of the scientist, the structure of the experiment, and the power of the data one can get a very good idea about whether or not some observation is random or not. How you interpret it will weigh in just as heavily (probably more) in how the "scientific" results are reported. In this case, new methods like new genetic technologies have completely changed the playing field.
Regardless, I disagree with the logical progression he reports for gay advocates. Genetic causality alone doesn't make something normal. Huntington's disease is genetically determined, but it's far from normal. In my view, it's usually an ill-conceived effort to try to ascertain whether homosexuality is genetic or developmental in cause. Nicolosi has based his therapy on the developmental hypothesis. Some gay advocates decry anything but an acknowledgment that orientation is innate and immutable. I think these people are grossly oversimplifying. I doubt there is a universally necessary cause for something as complex as sexual orientation (unlike chicken pox, for example, where the pathogen is always the same... and no, I'm not comparing homosexuality to a disease... right now). It's certainly not a matter of choice. At least, not in any definitive sense. Reparative therapy may be one choice that can influence orientation, and I suppose that's one reason many people hate it so vehemently.
When I first began this book, I thought Nicolosi's main flaw was that he believes one-size-fits-all despite his somewhat frequent (but seemingly contradictory) admissions that there are exceptions. Toward the end, though, he finally explicitly acknowledged limitations of his therapy:
Another type of client who does poorly is the one who does not fit the syndrome decribed in this book.... This type of client usually has no particular difficulties with male friendships or self-assertion, shows no evidence of male gender-identity deficit, and has a family history that does not fit our pattern. The treatment issues we address are not relevant to his issues.