If you want to argue base purpose, the base evolutionary purpose of pair bonding (which leads to marriage) is to ensure good child care for ones kids. Evolutionarily, men who participate in serial marriage, having kids with each, are very successful. Even though their relationships are not.

Evolution does not care whether your children are happy and healthy. It cares that you have a bunch of them, and they have children in turn.

I am not very successful evolutionarily. Nor do I strive to be.