So here is some backup on one of my possibilities. Mind you I still am not claiming that this is a cause yet alone the only clause of transsexualism.
Chimera (genetics) - Wikipedia, the free encyclopedia
Note the third example. If they can have partial sex organs of the opposite gender, what else develop in them that could cause them to be other than the obvious external genitals? Of course their immediate argument is that genetic anomalies don't count. I'm saying that unless you do the test to confirm or eliminate the existence of said anomaly within the transsexual, you can't claim that they don't have a genetic anomaly and are a straight (no puns intended) man or woman.