Lots of reasons are given as explanations: We live in a very different society from 30 or 40 years ago and there is no longer the same expectation or pressure for us to marry. We are no longer governed by religious codes of behaviour that said sex before marriage was sinful, and that children born outside wedlock were illegitimate. The law governing matrimonial property and child custody is now no different for people in de facto relationships compared to those who are married. And so on.
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Simply put, the institution of marriage is impacted by social change, and shows up in the stats.
However, once we are in a committed relationship for longer than three years, we are effectively as good as married under the law - unless we take steps to keep our financial affairs separate. Doing this creates some complex (but usually useful conversations) for couples embarking on a serious relationship, around harder topics like life goals and finances. Sharing a child is also legally no different for married or unmarried parents.
Our laws, then, have adapted to fit social change. But what about the psychology underpinning of all this? Does marriage no longer have meaning? Is it 'bad' for society that marriage rates are on the decrease?