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.