My theory is that newer generations have become disillusioned with the idea of monogamous, single-partner relationships. Simultaneously, concepts like "open relationships" have gotten more traction. And yet, marriage is a major part of most cultures (laws, tax benefits, religion, etc.)
So on one hand, you have things like marriage and monogamy, concepts that are perpetuated by society, who have deemed this the "right" way to live romantically. And on the other side you have the newer generations, who feel societal pressure to "fit in" but also live in a world where tolerance and open-mindedness are increasingly trumpeted as ideals, ideals that don't really jive with the idea that there is only one correct way to pursue a long-term romantic partnership.
Just off the top of my head, take it for what it's worth.
I actually think they want both though. They're conflicting and contradictory drives. In a natural more harmonious sense they aren't, it merely insures procreation, but in a world where we are completely self-aware they are at odds; one becoming detrimental to the other.
Religion seems to be the one most people talk about as being the reason for monogamy and marriage, but is it really that though? As I explained before, it seems to me to be rooted in the self-analysis and experience of basic drives. The dissecting and taking apart of our nature through experience and self-analysis. In other words, the basic desire of sexual gratification we call lust. The bonding and attachment to another we call love.
Lust we view as very primal, there is no right or wrong in it, it is merely a selfish desire for sexual gratification. I explored this in a very dark mind experiment. If you were stuck on a desert island with a female, where societies moral values and norms didn't exist, would you eventually rape that girl if she was completely uncooperative? Perhaps she is the most desirable woman you have ever seen, but she doesn't want to have sex with you. She is not interested in sex with you at all. It's not an easy question to answer is it? Simply because nature isn't about right or wrong. Lust isn't about what is morally acceptable, it is about selfish sexual gratification. From the perspective of nature it is only concerned with procreation.
Love in its broad sense is about the bond and attachment you have for another person. It can be quite selfless in that your concern is not so much about yourself, but it is about the other person. You try to make them happy, your protect them, you do what is in the best interest for that person. It is the root of basic morality. You deem what is right and wrong based on how it affects others. If someone murdered your children and wife in cold blood you instantly know it is wrong based on love. Love became a virtue, a principal for living.
Empathy is knowing and understanding the suffering of others based on your own experiences. This is also seen as a form of love in its broader sense. You know something is wrong based on your own self-analysis and experiences with suffering.
You can see how love became a model, a virtue for a greater standard of living. How morality formed on those principals. And how lust was deemed more primal, more animalistic.
Of course we all lust, we can't really escape that, it is more the fact that we gained a higher level of understanding. Greater more complex concepts were formed that shaped the reality in which we now exist.
Of course both are in our nature, therefore both manifest themselves within us. In fact both in some sense are interwoven. This sounds quite metaphysical, but through observation and experience we have reached a higher level of understanding/enlightenment on the nature of existence. We are on another plane of existence in comparison to other animals. This is because we became self-aware like Adam and Eve in the garden of Eden. This had both a positive and negative effect. No longer do we live harmoniously as we have realised we are not one but two opposing forces. This created a clash, a conflicting war within ourselves. However by becoming self-aware we gained a greater understanding into the nature of existence itself.