I've written in several posts about long-term relationships, what helps or hinders their survival, and how they are evolving. Is monogamy genetically hard-wired in humans, or is it a relic of church and/or state proscriptions? Why is there such variety across cultures when it comes to sanctioned marriages ~ variety in their duration and number during a lifetime, tolerance for infidelity, or even their presence (as opposed to simply living with someone as long as the arrangement suits everyone?).
Today I came across an interesting summary of our expectations toward relationships, in the past and projecting into the future. In Future of Relationships ~ Changing Views of Monogamy and Infidelity, biological anthropologist Helen Fisher suggests that "marriage has changed more in the last 100 years than in the previous 10,000, and it could change more in the next 20 years than the past 100. W'ere shedding traditions that survived since our culture began .... and it appears we're returning to the ancient sex and romance practices of our hunter-gatherer days." The article goes on to flesh out our ten-millennia history of partnerships, with special attention to how all that has changed over the last century.
Looking forward, futurist Sandy Burchsted proposes that "in the future, most people will marry at least four times and experience extramarital affairs with little public censure. Marriage will be considered an evolutionary process, not a one-time-only event. The first marriage will be seen as the icebreaker, lasting about five years, where couples learn to live together and become sexually experienced. But once disillusionment sets in, it will be perfectly acceptable for the couple to separate, as divorce will carry little stigma in the future. Next, people will marry for a 15-to-20 year parenting experience. Raising children will be the primary purpose for this arrangement.
"The third union, called self-discovery, will be about partners getting to know each other at deeper levels and better understanding what they hope to gain out of life. The fourth and final marriage will be a late-in-life 'soul mate' connection, filled with marital bliss, shared spirituality, physical monogamy, and equality. With medical science extending lifespans, this marriage could last indefinitely. The perfect, deeply rewarding relationship that humans have always been pursuing, seemingly forever, might finally become reality."
It's an interesting prediction, but all predictions are fraught with uncertainty. Humans being the diverse and evolving lot that we are, I doubt that everyone would agree to Burchsted's scripted four-marriage schema. In fact, many young adults are postponing or discarding marriage entirely, opting to simply live together, have children together when the time is right, and live their lives outside the framework of formal marriage. If it works for them, great. I agree that having relationships outside the primary relationship may become more common, though I can tell you from experience that unless everyone involved is fully informed and fully comfortable, and unless all agree that the primary relationship comes before all others, the arrangement becomes unwieldy at best, destructive at worst.
I doubt that any approach is right for everyone, whether within a culture or between cultures. What's called for is a high degree of tolerance, a willingness to consider the merits of new ideas, and unwavering respect and caring for one's partners needs ~ and one's own needs as well. It is appropriate to question assumptions. It is also appropriate to do so mindfully.