I don’t remember seeing rules in the dictionary definition, and I also struggle to believe that something can’t be sexist/sexism if the laws allow it. I think in the western world we would say that another country banning women from driving would be an example of sexism, albeit within the laws of the country.
Also, I hate to break the illusion for you, but boy bands are often planned as such and are manufactured by the record labels. It’s not a coincidence, for example, that the spice girls are all girls - that’s because they only auditioned girls because they were making the spice girls.