Lisa in Waukesha, Wisconsin, has two Facebook accounts. One reflects her liberal politics; the other is for acquaintances and family members to whom Lisa shows only her cat photos. Christina, in Milford, Massachusetts, has a sign in the back window of her car proclaiming support for a Democratic candidate. But as soon as she parks in the company lot, she puts it facedown on the backseat. Byron has lived in the same small town of Pomeroy, Iowa—population 662—his entire life. He brings his partner to family dinners but has never actually said to his conservative sister that he’s gay. Lisa,...