Oh, I'm just thinking of school and sports/changing rooms and doctors and records and passports/other ID and school... And digging a fraction deeper, there's the fact that communities aren't so isolated and unknown these days - an awful lot about Gwyna could be explained as her being from Not-Here. Plus she had Merlin (can't begin to attempt spelling of his name in the book) to smooth away all difficulties. Though I did amuse myself rather by imagine Peter Mandelson telling Tony to disguise one of his boys as a girl.