It's certainly true that married and single people socialize differently. My mother noted that after her divorce, she got fewer invitations from her married friends, and this wasn't because they took my father's side. But what we have here is a club of friends who are married, not a club of which all married people are members. In a typical women-only space, all women (by whatever their definition of "women" is) are welcome, even if nobody else there has ever met them. That's what gives it its character and makes the definitional question important, because they're there in their capacity as women. The married friends who socialize without single people are there in their capacity as friends, even if their single friends are excluded.

True enough also that the existence of same-sex marriage changes the idea of what it means to be married in these folks' minds. But nobody's ever argued that point in court, because it's so transparently flimsy a basis to argue that this means that other people shouldn't be allowed to get married.