I think there’s a difference between a group that forms to provide an environment of common interests and needs of support on a particular basis and one that has a distinct policy of excluding people who do not belong to the group for a discriminatory reason.

Does this organization expressly exclude people who are not gay? Or does the organization welcome anyone interested in joining who wants to support its cause? If it’s the latter, then I think the organization is simply a special interest organization for those with a common interest and identity and not a discriminatory one.