Danimay, to use an imperfect analogy, think of marriage as a building. The newly wed couple moves into a house. They live in it thinking it’s their house. Now, a few things could really be going on. They could have bought a house that did not have a free and clear title. So they are occupying a house illegally that belongs to someone else. When that comes to light, they can be told to move out. That’s the kind of marriage where the people are in it and someone has a vow to someone else and is not free to marry.
Or they can move into the house, and think they are happy in it, but not realize that the house is so structurally deficient that it is only a matter of time before the building comes crashing down around them. It doesn’t matter how happy they were in the beginning, if the floor was rotting through and the foundation was cracked, eventually the house will be a hazard and they may have to move out to save their safety. And a building inspector coming in later will tell them that house was never fit for human habitation to begin with. And it doesn’t matter how happy they were with the plastered over wall cracks and the new carpeting hiding the structural defects. The house was fundamentally unsound.
A house that lasts is proof of its soundness.
Same with marriages… marriages that last, whether blissful or not, are fundamentally different from marriages where one or more persons leave, or abuses, or is unfaithful or doesn’t intend to have children. It’s like a building that crashes in on its own imperfections eventually. Some weddings never should have taken place to begin with.