Given a choice nobody will marry an ugly person. It might be an insensitive statement to make, but is it not true? So what would you expect to happen to a relation which is based on physical attraction to begin with? It will be as fickle as the attraction which started it. But we fight with it day in and day out. To add some substance to it. Kids are of course a game changer. But what of a long and successful marriage without kids? In the end what will become of the two people involved in it? When none of them is attractive anymore.
Actually the end is easy. It is the middle years when all the infidelity and such-likes happens. When both think they deserve better. It is like marriage is designed to ruin you. But somehow, by a stroke of luck, you might be saved. The more successful and genuinely happy marriages that I have seen look more like any other friendship rather than the added baggage that is part of the marriage bandwagon.
But like any other hard, near impossible thing (like rock climbing, deep sea diving, cliff jumping) marriage, if done right, can forge character. Of course it might be a tad difficult if the spouse is a psychopath. But in most cases they are not. What remains is letting go of your ego and being kind. Ah, but what seems clear in writing, is hazy in doing. What seems like an obvious thing in principle, is always the hardest thing in practice.