as said above, assaults are more opportunistic than planned. they target females walking alone etc, not in a public space with potentially half a dozen other women present ready to rip their bollocks off if they tried something.
besides, if someone attacked a woman in a toilet or changing room under the ruse that they identify as female then the mail, sun, express etc would be all over it. i've seen nothing of that sort so it's obviously not a common occurrence anyway