Robert Silverberg. The stochastic man (№ 355359)

Need each other is one thing, but monogamy is quite another. And if the first still exists, though, and undergoes evolution, the latter is something like the Dodo or trilobite.
But monogamy is one thing and chastity is another, and if the former still exists, however metamorphosed by the evolutions of the era, the latter is one with the dodo and the trilobite.