Earlier quoted context omitted.
Gender discrimination is obviously not illegal. How do you think rec sports leagues or support groups work?
Gender is a protected class, so "it's (not) illegal" in either direction is a bit of a blanket statement. So let me try and be more precise. In some states, AFAIK the "women-only gym" is allowed, but in others it isn't. The point, if I remember correctly, is "exercise requires such compromising clothes and positions, that the right to personal privacy trumps the right to not be discriminated based on gender". And eve…
> Gender is a protected class
Which means that businesses doing public accommodation are restricted from discriminating, but 'private clubs' (such as the Boy Scouts), who have a narrowly tailored interest that is benefitted by discrimination against a protected class is generally allowed (subject to variances within state law).
For example, a group for men who have been sexually molested by men would likely be considered presumptively lawful, but a group for men who also happen to be car salesmen would likely not be.
There are a number of details that need to be considered as well, as (loosely) prescribed by _Rotary Club of Duarte[1]_, such as exclusivity (it can't be publicly available to be overheard), and purpose (already covered), but if neither conditions are met, and a state law prohibits such discrimination, then it meets (at least) a rational basis standard.