If the psychologists say something scientifically demonstrable then they are experts. If they simply say "homosexuality is a normal variant" then they are expressing an opinion of no more value than mine or yours.
There is no body of knowledge that makes anybody any more expert on what is "normal" or "abnormal" than you and I can examine, right here on this thread.
Do you really think that the APA just decided one day to say that homosexuality wasn't abnormal without any scientific evidence to back it up. If they can't measure normality, than the disorders they have diagonsed and labeled aren't worth anything.