From Daniel Jones' Q&A about Modern Love over at the NYT.
Some people think we live too long to commit to one person for life. Monogamy may have made sense a few centuries ago, they argue, when we tended to die in our 40s (after raising a dozen children). But being with the same person well into our 70s and 80s? That simply can’t be natural.
This is a question, by the way, asked almost exclusively by people in their 40s (or younger). People in their 70s and 80s do not ask this question. They are, by and large, very happy to have shared a lifetime with the same person.