Marriage
Monogamy, that's why.
It's simple: the monogamy model, or more correctly, the scarcity of love model - or dogma - that is popularized by Hollywood and the church (talk about an unholy alliance), increases the stakes and reduces the odds of winning to such a degree that most people instinctively grasp just how slim those chances are, and just as instinctively, they refuse to make a bet.
And even when they do, finally, relent to make the bet, it obviously fails with a high degree of certainty. Divorce is simply the more flamboyant of failures, but broken marriages full of resentment abound, not to mention marriages that are open in anything but name, those marriages where one or both of the spouses cheat repeatedly. Which is sad, if one gives it a second of rational thought: if only the spouses agreed to open their marriage, neither of them would be cheating anymore, and some of those marriages might become happier (while those that take a turn for the worst would not have been happy nor saved anyway).
How many times have I heard "we'd been married for years, and I woke up one morning and I found a different person, and I still love them deeply, but I can't live with them anymore"? of course this happens. It happens all the time. In fact, most people will simply shrug it off with "people change". But think of the loss when this marriage ends in divorce. The couple still love each other, deeply, but they can't bring themselves to agree that they have both changed - as people are wont to do - and their needs are not entirely met by the other. Interestingly, they will usually both admit when pushed that some of their needs are met by the other in a perfect way, a way that no other could match - and that makes sense, too. Why destroy so much good, an entire life of understanding, and love, and partnership? that's, after all, what your pastor will tell you. I'm telling you the same, but with different conclusions.
The prevailing view of monogamy, mind you, is not just about sex. It is about "fidelity", that concept that supposedly means that one person's physical and emotional attachment needs must be fully and entirely fulfilled by exactly one other person and no other - and for this to hold true for 40, 50, 60 years until they die. Yes, this might work for static folks, and indeed, I have met a few couples in my life who were perfectly happy with one another, but they were amazingly even-keeled, unnaturally "plain vanilla". But dear god, these people are always so... darn... dull! the quality that makes them happy with the monogamy model is the same quality that makes them quite content with having life be no more than a single smoothly sailing ship that never wavers as it sails slowly and surely on a straight path, for decades and decades circling an endless globe full of gentle water.
Personally, I can't imagine living this way. There's no passion in this sort of relationship. And most people I've met in my life are passionate about something, usually more than one thing. A quick discussion about politics or money (in the US anyway) will immediately tell you the sort you are dealing with...
Thus it is no surprise at all that as the increase in personal and individual freedoms picks up in the western world, so does the rate of failure of the monogamy model. Monogamy, by definition, is based on a controlling societal and cultural structure, one that makes a tremendous number of assumptions, chief of them being that people can and should control their own happiness in order to fit the model. This works just fine in a world with less freedoms, where people are used to not exploring within themselves but rather to fitting the societal mold. It breaks in a hurry in a world where people are encouraged to love themselves, to look for their own happiness, to develop and grow and expand their horizons, and where they actually have the time and resources to do so (we are quite privileged to be living in the US of 2008, by the way). Inevitably, a dynamic person - as most of us are - will find that what worked for them yesterday may not work today in exactly the same way.
That leads to the serial monogamist, the most typical of today's singles, the one who is monogamous for a short amount of time with each partner. It leads the the notion that a new relationship only become serious when sex becomes "exclusive", but with the tacit yet never spoken understanding that this is merely a symptom of love itself becoming exclusive, an almost preposterous notion even to the monogamist who isn't married yet (and to most who are). And it leads, of course, to where a person growing up with the monogamy model seared on their brain cannot for the life of them understand why they can't "make a decision". Of course they can't. The stakes are too high. What if that person can't fulfill the dream of decades and decades of emotional, mental and physical fidelity? it makes no sense that they could - the odds are so low - so folks find themselves unable to commit, finding little faults which are sure to cause a breakup in the future. Or losing the commitment once married and finding little ways to feel freedom again, whether it is by sleeping with others, or finding and chatting with opposite-gender friends online (the gay community, as they tend to always be way ahead of us straights, had long since figured this out), or having little flirts with work colleagues, or going to titty clubs or in hundreds of other ways.
This is a getting a bit long, although I can say much more about it, but I'll end with the answer. I think you know what it is already. Polyamory (not to be confused with polygamy or swinging). Open marriage. It makes so much sense it scares a lot of folks who like to cling to tradition as their shield. But I'm convinced it works. Conceptually, at least, Heinlein had it right all along.