So labelling anything where two genders are treated differently as 'sexism' or 'sexist' I don't think actually matches the modern usage of the word. I think the difference is it's usually used in a negative connotation and the type of discrimination is seen as non-acceptable - for instance most people wouldn't call a girl-band or boy-band sexist because they select their members based on gender, while most would call an employer sexist if they had a generic business and tried to segregate their teams into single-gender teams. Most people still don't have a problem with boy bands (i.e. a male-only-team in a music workplace), thus not sexist, but do have a problem with male-only-teams in other workplaces, thus sexist.
Enforcing a rule isn’t discrimination. The rule itself may or may not be discrimination.
> Most people still don't have a problem with boy bands (i.e. a male-only-team in a music workplace), thus not sexist, but do have a problem with male-only-teams in other workplaces, thus sexist.
They get the label “boy band” after they form. If they were a mixed gender group (like a workplace) and kicked out a talented female musician because they wanted to be male-only, that would be sexist.
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.