I wanted to throw a few sociological observations in here.
In my rather humble opinion, and in counter with the usual blathering about the “sanctity and constancy” of marriage, the thing is that this humble and well-regarded institution is always, and always will be, in the process of evolution. Marriage until fairly recently wasn’t considered a romantic pairing as much as it was considered a financial pairing, and extended families were far more common than they are now. The pressures for a couple to stay together are much less. Additionally, the relative economic liberation of women (at least in the first world) and the access to birth control have made marriage an option rather than an obligation for women.
Additionally, the younger the person getting married, the higher the tendency to divorce later on. In your 20’s and early-to-mid 30’s, things tend to not only be economically unstable for a couple, but they also tend to be hormonally unstable. I’m not saying that all couples will divorce in that age range, what I’m saying is that the conditions that promote marital problems are going to be a bit more present during those decades.
So I don’t see it as laziness, immaturity, godlessness, or some kind of indication that society has gone to hell in a handbasket as much as it is obvious that society is changing, and marriage is rather slow in catching up to these changes.
My observation is that the older a person gets, the more stable their romantic relationships become. I think by then we have a much more pragmatic view about love and long-term relationships, and understand our own as well as our partners foibles, and are a bit more forgiving of them. Additionally, not all marriages are monogamous-- I’ve seen a number of successful, loving marriages that are either openly or, more common, quietly polygamous. The reason they work is that the people involved have rules about it, respect them, and are open to re-negotiating with their partners. It’s all a matter of shifting perspective.
My partner and I have been wildly happy together for 11 years, and the reason it has worked for both of us is that we both love each other deeply, and we trust each other. It also helps that we both agree on money, know how to talk respectfully to each other, have pretty equal sex drives, and most importantly, know the other loves them back. It took a few relationships to learn how to do that, and to learn how to look for a person I wanted to spend my life with. All relationships have a learning curve to them, and marriage is no exception.