The notion that a pair bond should last a lifetime is a popular idea, and it's the theme of many romantic stories and songs. For the majority of people, however, there are several mating relationships of varying duration over the course of a lifetime.

A subset of the population does manage to bond with a single mate for life, but there are also others who mate with dozens of partners.

It is clear that when a couple is in a long term relationship that they don't magically stop being attracted to other people. The evolutionary advantage of mixing the gene pool is a strong one. I don't know whether we are evolving towards a greater degree of monogamy, or whether we have reached an optimal level.