Taylor Swift has been writing songs about marriage for as long as she’s been famous. “Take me down to the time when we walked down the aisle / Our whole town came and our mamas cried / You said I do, and I did, too,” she sings, on “Mary’s Song (Oh My My My),” a track from her début album, released in 2006. Swift was only sixteen when she wrote the tune, but, in some ways, it is the paradigmatic Ta...