There is a saying attributed to Groucho Marx: Marriage is a great institution - but who wants to live in an institution?
In Britain, divorce rates are falling. The reason for this is that it is now acceptable to live together and not get married in the first place. Some people still think this is terrible, but more don't. Most think it is just sensible and realistic. Arguments on this issue tend to revolve around whether the protagonists are thinking primarily of the wellbeing of the adults or of the children. However, arguments on both sides tend to lack subtlety and realism. A great many couplings these days are childless. Also, it is hard to see how children benefit from being in loveless families. However, this does, in turn, highlight the fact that families with children are a rather different species from families without them and with increasing longevity, nearly all child-rearing families do become childless families in due course if they last that long. One wonders if the law should be different for the two types. Or one could go further and ask why the law is involved at all. Is it not up to individuals and their social communities to sort out what kind of relationships they want to have? What has this got to do with the state?
In the contemporary West we see the state not merely as the regulator of rational social administration, but also as a culture forming device. A certain kind of monogamy is not simply the preferred option of many people, it is also seen as something to be imposed upon everybody who does not remain single, whatever their private views on the matter or, as in the case of some immigrants, their background culture. Nonetheless, if people really want to flout the established system it is just about possible to do so so long as all parties are firmly in agreement.
What would happen, however, if all laws pertaining to marriage and divorce were taken off the statute book? The question that immediately arises is: who, if anybody, would suffer? The first likely answer is children, but is this so? One imagines that most families would form and conduct themselves much as now. They would do so voluntarily or in conformity to the customs of their culture or religion. Some would endure and some would break up, just as happens. The question is not one of marriage and divorce, but of how to protect children and give them some rights at law vis-a-vis the adults who have responsibility for their care and protection. Clearly there is a place for the state and the law in this area of children's rights. If adults neglect or mistreat their children there are consequences, whether the adults are married or not, and, arguably, it should really make no difference whether they are or not - the child still has the same needs. The argument would then be that it is easier to tie down who is responsible when there is a marriage, but since cohabiting is becoming as common as marrying these days, is this a realistic argument?
It seems that marriage continues because at least some people want it. It is simply supply responding to demand. Even gay couples want it. It gives a certain status and provides a background of taken-for-grantedness that simplifies discourse within the relationship. The belief that infidelity will never happen is fiction, but if the fiction is given lip service by all parties, then it is true until it isn't. It is difficult to get accurate statistics on infidelity for obvious reasons, but there is reason to think that the estimate of 50% of married people being unfaithful at some time during their married life will be on the conservative side. Just because people have got married does not mean that they stop assessing other people they meet as potential partners. It is commonly reported that men cheat more than women, but this is questionable. The same surveys tend also to show that all woman have less sexual partners than men, but if this is so, who are the men doing it with? Human behaviour is a function of some equation between various variables. These include opportunity, social pressure/permissiveness, confidence, attractiveness and so on. Nature has equipped us with drives that seek the best reproductive bargain, whether we actually reproduce or not.
Modern life has changed some of these factors in a variety of ways. Modern people often spend more time with work colleagues than with their spouse and nowadays colleagues may be of the opposite sex and, by dint of being colleagues in a similar occupation, are likely to be people of similar age with similar interests. Again, the internet has made clandestine communication easier. On the other hand, modern life is more public and visible than formerly and the internet also means that if one's spouse is away it is still relatively easy to communicate every day rather than waiting a week for a letter.
I rather think that it probably does not make much difference what the law is or says, people will do what they do. Couples will form and some will break up due to their internal dynamics and others due to being superseded for one or both partners by what are experienced as better options. Children will be born and cared for, either by parents who stay together or by ones who part. Some parties will be amicable and others consumed with jealousy and bitterness, and everything in between, and the law is not likely to make much difference to this, except insofar as it is used as a lever by one or both parties to get revenge or compensation or protection therefrom.
It is the use of the law as a lever that makes one doubtful about its involvement. Many couples must actually get married because one partner wants to and the other feels unable to refuse. This really is the sense of Marx's joke - marriage can be a cage. The partner who wants it is likely to be the one who feels less confident and seeks to tie the other partner down. Such coercion, however, does not ensure real love and may be a recipe for enduring misery.
I imagine that, realistically, marriage and divorce law will continue and that it will, with some time lag, continue to reflect common or majority mores, but, while it may still have a dynastic function for the aristocracy and super-rich, legally binding marriage is probably really a rather unnecessary bourgeois institution encouraging more hypocrisy than liberation. The people who are happily married would be happily together anyway. The ones who are unhappily together would still have to face invidious choices. The children involved need protection, but while this can and should extend to ensuring their physical care, one cannot legislate for love. Religions have always blessed unions and would continue to do so whether the state was involved or not. It is basically a good thing that social attitudes have shifted and there is now little or no stigma attached to people living together without legal binding. This more permissive situation is altogether more compassionate and realistic.