Marriages can in fact last forever. However, to do that, the stagnation traps that plague so many of them have to go. Between stagnation traps at work and regulations preventing couples from being free to express their feelings toward one another, marriages are doomed to fall apart. They get in ruts, which is the root of most marital trouble. Once they get in ruts, it is no longer exciting, and they start arguing about petty things. Eventually, adultery results or differences that are not repairable come up.
However, if this stagnation is eliminated, the marriages are no longer prone to get stuck in ruts. If it remains exciting throughout life, such differences will not come up. (Or if they do, they are easy to settle.) Money would be abundant, and work exciting (eliminating money arguments). And the excitement remains--adultery usually happens when one or both partners get stuck in ruts, and if those ruts are not there, most adultery will not happen.
I might add that Jehovah's Witlesses have the highest stagnation of any major group I have seen. They have crap jobs (even worse than the ones that most people have). They make pxxx poor wages. They waste lots of time in field circus, where intimacy is banned. They waste a lot of time presenting the same littera-trash at each door, when anyone is home (and it is a carbon copy presentation at every door). They are regulated as to what they can and cannot do in bed, plus their time is for field circus or studying the Washtowel or Kingdumb Misery. This is adultery waiting to happen.