Curves is a Christian gym for women, let them be, they've never done anyone harm : )
Two things: First, I never realized that Curves was a religiously affilliated organization, I've not seen that in their advertising. Second, if I started a men's only gym, where successful businessmen were the only ones allowed in, there'd be a cease and desist order nailed to my front door in a week. Some feminazis would find an ACLU'er to charge that I was trying to keep women out of the loop, and discriminate against them by perpetuating an "old boys network".
I don't really have any problem with Curves being women only, but it seems hard to justify under current interpretations of the law. I did do a bit of research since my original post, and found that Wisconsin specifically passed a bill allowing same-sex health club facilities, wonder if it would pass muster with a state's ERA.