Marriage is not only the result of love, but also the result of social income and reproduction.
I think human beings are designed to live in the wild and try their best for food and reproduction, just like other animals. Having marriage to limit the natural desire on other woman is not natural. However, human beings concern about marriage in order to gain benefit.
In the past, woman had less income than man. Therefore, the livelihood of woman could be more secure by marrying to a man. And the society had less support to the poor. The children grew better with both a father and mother. Therefore, marriage did have its function.
Nowadays, woman can have more income than man. Couples may not give birth to baby. Children could be brought up with single mother or single father. Therefore, marriage seems not so suitable to modern society. No doubt about the devoice rate is increasing.