Intimate relationships as our grandparents knew them, whether platonic, romantic or purely sexual, don’t work anymore. A quest for efficiency has powered society’s economic miracles since the Industrial Revolution, but the accelerated, relentless 20th-century streamlining of our lives also made us too busy for everyone but our nearest and dearest, and sometimes even for them. In recent decades, marriage rates in rich countries have plummeted; having grown up in a milieu of unhappy marriages, young people are increasingly saying “I don’t.”

The story is more complicated than that, though. Just because many of us aren’t marrying doesn’t mean we’re not committing. Today we feel freer than ever to pursue our sexual desires—a path that was largely forged by last century’s LG...