Oxford definition of “sexism” via Google:
> prejudice, stereotyping, or discrimination, typically against women, on the basis of sex
The definition of sexism seems to include discrimination. What definition are you using?
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.