The problem is with the failure of the culture to take marriage itself with the seriousness it used to receive. Connected more with pleasure than with responsibility, marriage functions more and more as just as a set of personal preferences. If it feels good, tastes good, looks good, etc., -- hey, why not? If none of the above, never mind. Each to his own. It 's one big case of the modern moral disorder, the one known as ... "whatever."
Time was, and it wasn't long ago, when marriage, as developed and refined by belief and practice over many centuries, was the norm for human beings. Admission to the institution was generally a sacred rite, protected, furthermore, by the laws of the state. Once entered upon, marriage tended to last, which isn't to say that the initial rapture of the partners grew or just endured. Not infrequently, such is the nature of humans, did it wither. The importance of the institution was nevertheless a rule of thumb. A very big deal was marriage: It still is, in the eyes of many -- just not so many as was the case up until half a century ago.
What changed? Charles Murray ("Coming Apart: The State of White America, 1960-2010") remarks that sexual revolution became inevitable once oral contraception came on the market. "Of course, sexual mores would be profoundly changed when, for the first time in human history, women had a convenient and reliable way to ensure that they could have sex without getting pregnant, even on the spur of the moment and with no cooperation from the man." Marriage's ancient purpose in the projection of new life lost resonance. The Supreme Court's whole-cloth creation of a right to abortion further degraded the instinct to bring forth and nurture children. The little brats -- who needed 'em?
Not the culture of self-esteem and self-indulgence that took over America in the '60s, '70s and '80s, intent on establishing a right to self-expression and personal fulfillment, never mind whole libraries of ancient wisdom and admonition.