@Setanta,
One of the reasons for that, and I agree it is embarrassing and awful for an older boy, is fear of the male sex offender abusing a boy when he is alone in male toilets etc.
Unfortunately, this happens often enough to make it a not un-reasonable fear.
And, as far as anyone is able to tell, there ARE less female sex offenders.
That doesn't mean that one is not aware of and protective re the possibility.
It's a dilemma here when I take young male clients to the loo.
If they are comfortable to use the women's, I accompany them into the female loo...not into the cubicle of course!
If they are not, we see enough kids abused by strange men in toilets, that I actually check the male toilet is empty, and then wait outside the door for the client.