By abandoning the norm of marriage we end up with fewer children. This ultimately leads to economic crisis, as fewer workers translates to reduced growth and not enough tax revenue to support social welfare systems. Governments may try to mitigate this by taking in immigrants to replace natives who refuse to have children, but this solution too involves a number of trade-offs. A nation may get some cheap workers to bolster the economy. It may also get more economic inequality, the destruction of working class industries, ethnic strife, and eventually cultural dilution and degradation.
In theory the government could drastically cut spending and the wider society could choose not to take in immigrants and instead adjust itself to a reduced population without economic strife. This is extremely difficult but technically possible. The problem is that it is at best a temporary solution. Japan for example will see it’s population reduced by half by the end of the century. Eventually, the society will simply cease to exist. Adjusting to a shrinking population is the equivalent of a society rolling over and dying, but making itself comfortable first. If a civilization seeks to have a future, it eventually has to solve the problem of birthrates and get to a replacement level.