I think marriage is an artificial social contrivance. I've been married and it was totally artificial. Consider all of the things we humans have evolved to do, including reproduction. It takes a village to raise a child, but there doesn't seem to be any mandate for a one-on-one arrangement.

I've been married twice but my 2nd marriage which made me a widow is one of those I think I could have been happy with for a 100 years.

I always felt it was a success because both of us cared enough about each other that our goal in life was to make the other person happy. Even after sickness arrived I could not imagine not wanting to do everything I could to make him happy. I am far from a Saint but I think if people are lucky enough there will always be people that can maintain a marriage as long as they're alive.

There have always been and will always be people that have very different goals for relationships. It is definitely true that alternate lifestyles are much more acceptable than they were when I was younger but there were weird things going on when I was younger as well.

I have several married friends that amazingly still run around with me. I'm sad to say that out of say 5 couples only one of them has what I consider a truly loving and satisfying relationship. When I look at the four couples that are in less than satisfying relationships, it seems obvious to me that in all of them one of the partners is not that interested in making the other one happy. Two of the guys obviously want to experience sex outside of their marriage and their 2 wives are aware of this and anxious about it all the time. The other 2 obviously love each other but are not willing to change in any way to make the other person truly happy. Consequently those 2 couples keep having on going battles and just tolerate each other most of the time.

I imagine there are some (very rare) people out there who would stay married for over a hundred years, but people evolve and change over time so it seems unlikely to me that maintaining the same interests as another for that long is viable. Many women have spent a long time finding a way out of the restrictions of a mandated long-term marriage. The idea of a contract with an end date is intriguing - maybe with a renewal clause?

I see fewer and fewer marriages these days, so the institution is already evolving.

That is a very interesting question. Children do need both male and female role models, so the future will provide some, but marriage isn't a requirement. People need mates psychologically, so the future will provide. However, the economic needs may become moot, except some people need power enough to risk their lives. These power mongers will attempt to maintain economic dependence, because it gives them control over peoples lives. They will use force to maintain power, too. The course of human culture is affected by many things, too many for me to understand. However, change is inevitable as death.