I really don't understand how people can take the position that protecting SSM is government encroaching on marriage, or telling people who they can or cannot marry. Protecting SSM is the government divesting itself of the ability to prevent people from marrying. It is the government saying "you know what, we're not actually allowed to pass this law and tell people what to do." Protecting SSM is getting government farther out of marriage.
Locker rooms and bathrooms are more mutually agreed upon, so the same standard might not apply, but they also serve a purpose beyond mere separation, that is, protecting women from rape by giving them a safe place to do those things. It is not that discrimination is always unconstitutional, but that it is prohibited without a compelling purpose. The same is true of sports teams. Separate teams for males and females is intended to promote fairness, similar to weight classes in boxing.
I'm not aware of any jobs with separate standards between genders where appearance is not a part of the job (which is why you won't find male waiters at Hooters, for example), but if they are, they would be discriminatory without some compelling purpose behind them. Separation for the mere purpose of separation is unconstitutional.