What's the name of the book/short-story sci-fi anthology that contains this story about aliens saving two high school humans just before an Earth-destroying event (possibly nuclear war or some other catastrophe)? The aliens are trying to preserve the human species by getting one of both sexes but they chose based on clothing (because they don't understand enough about human gender) and both humans turn out to be girls.
It might be Roger Zelazny's short story Fire and/or Ice. It starts with –"Mommy! Mommy!" –"Yes?" –"Yes?" –"Tell me again what you did in the war." The premise is that Götterdämmerung brings the end of the world, and a being that calls itself Loki saves two human survivors, but doesn't realize that both are girls because one is dressed as a man. It turns out that both are pregnant. "And we both had twins, and lived happily ever after."
The story is the first of the triplet Fire and/or Ice, Exeunt Omnes, A Very Good Year, originally for Robert Sheckley's anthology After the Fall (1980), and republished in Zelazny's Unicorn Variations (1983).