You are aware that this case was settled 12 years ago? I dont see a mass exodus happening anytime soon.
First off, this isn't a legal issue, as the organization is free to practice discriminatory policies according to law. This movement isn't challenging that. What it does is challenge the organization itself, who very recently adopted a hardline stance against any gay or lesbian leaders or members. Basically they went from a 'don't ask, don't tell' type policy to a more openly discriminatory one.
Also, society is clearly changing, and this type of discrimination is not just being fought by gays and lesbians. This is a lot of negative publicity for BSA, I have a hard time believing it won't hurt their membership down the line. I have quite a few friends who aren't just turning their back on BSA for their children, but jumping to support groups with non-discriminatory policies. Add this to the fact that Eagle Scouts, which are extremely highly regarded, are stating to protest this move, and you've got a very bad situation for BSA.