The theme to the 1970s sitcom All in the Family, "Those Were the Days," is a lament for a time when, supposedly, a clear sex and gender binary existed and everyone conformed: "Girls were girls and men were men." What does the science say about these binaries? Are they real? Universal? Unchanging? In this series of three episodes, I explore three common arguments in support of a binary view of sex and of gender: Sexual reproduction requires two sexes contributing in unique ways (Part A); there are obvious physical and behavioral differences between females and males (Part B); and human societies are built on a binary female/male foundation (Part C). What does the evidence say? Are these binaries indeed "facts of nature," and if so, to what extent?