Two hundred years ago, few people foresaw legalized divorce or open homosexuality—let alone gay marriage. Abstract art and jazz were unimaginable. Aesthetics, morals, and family relationships, it seems, are the bane of the futurologist. We constantly speculate about the future balance of power, looming conflicts, and emerging technologies. Yet somehow, we imagine that morals and aesthetics are immutable. So we forget to ask how conceptions of good and evil, acceptable and unacceptable, beauty and ugliness will change. And they will.

Monogamy, which is really no more than a useful social convention, will not survive. It has rarely been honored in practice; soon, it will vanish even as an ideal. I do not believe that society will return to polygamy. Instead, we will move toward a radically new conception of sentimental and love relationships. Nothing forbids a person from being in love with a few people at the same time. Society rejects this possibility today primarily for economic reasons—to maintain an orderly transmission of property—and because monogamy protects women against male excesses.

But these rationales are dissolving in the face of powerful new trends. The insatiable demand for transparency, fueled by democracy and the free market, is placing the private lives of public men and women under greater scrutiny. The reality of multiple lives and partners will become more apparent, and society’s hypocrisy will be revealed. The continued rise of individual freedom will permanently change sexual mores, as it has most other realms. Likewise, jumps in life expectancy will make it nearly impossible to spend one’s entire life with one person and to love only that one person. Meanwhile, technological advances will further weaken the links between sexuality, love, and...