Much has been written about privilege in academic settings over the past few decades. There’s the privilege of wealth, and the advantages wealth confers if a baby is lucky enough to be born into it. Much too has been written about the advantages of being born into this world as a Caucasian — known in academia as “white privilege.” But not enough has been written about the most important advantage a baby can have in America: the advantage of being born with a mother and father who happen to be married. Call it “the marriage privilege” — the advantages are...