Not true.
As bluecan pointed out not true for every fish, although that is only true for hybrids.
Parrot fish can be sexed via the appearance of the genital tube, and I am not referring to a fully dropped (as they do prior to breeding) tube. All of the comments in this thread sound almost word for word like the ones in my last "please help me sex these parrot type fish" thread.
All four fish in that thread were fairly accurately sexed by 2 (or was it 3?) different people. I'd have to check to be sure but I think it was 3. I can sex them at the 4 inch (approximately) size range, even with only a barely visible (non breeding condition) genital tube, but as I said before, unfortunately I am not good enough to do it with fish this small. Mostly due to the fact I'm blind as a bat, lol!
I do have a few guess's, 1 and 6 are possibly female.