How would Mary...
...be able to describe (even though it was discreetly performed as whispers her grandmother's ear) Karen and Martha conducting lesbian sex? She saw Karen kiss Martha on the cheek and looked shock by it but surely she wouldn't have the knowledge of what lesbianism is or what lesbian sex would look like. It seems a little unrealistic to me.
However, I still like the story. It accurately depicts society's negative and destructive view on homosexuality at that time. Although, I am disturbed by what happens to Martha, I still think it's a well-constructed story for the most part.