People seem to be operating on the assumption that what goes on in the Guides is equivalent to what goes on in the Scouts is somehow linked. It isn't. They are entirely separate organisations and make their own decisions.
The Guides haven't been 'allowed' to be girls only for some particular reason. They don't have to legally provide any justification at all. There is no legislation that forces a youth organisation to admit both genders or 'allows' them to be single sex. The Scouts made a decision that they were going to admit girls and the Guides did not make the same decision about boys. They are entirely separate and do not have to make decisions based on what the organisation does.
If you want your son to join a single sex organisation, such organisations exist, such as the Boys' Brigade.
If you want your son to join an organisation that admits both boys and girls, such organisations exist, such as the Woodcraft Folk.
If you are unhappy with the structure, organisation or availability of existing organisations run by volunteers, you can set up your own club and no law will stop you from making it single sex or both sexes.
As for people complaining about gender in ballet classes. Have you ever actually seen a ballet? The dancing is highly gendered. If you don't want your son to dance differently to the girls, put him a class that teaches a style of dance other than ballet, of which there are many.