The answer is clear from a Darwinistic perspective. It is even intuitively obvious. In general, women want someone with resources and good genes. Since few men have both, women emotionally flip-flop between the two drives over the course of the month. Animals in nature have the same priorities, and human animals are no different.
Men rarely possess both attributes. Therefore, most single women will have children with the man who has good genes during their proliferative phase and then seek resources from someone else. This is because most women cannot get the man with good genes to commit to one female forever. In fact, these men have the incentive to spread their genes around. These are biological urges.
This also explains why most single women vote for social services and welfare. It is a way of getting resources from the men they do not want to immediately reproduce with. Patriarchy intentionally prevents this from happening by controlling female sexuality and limiting it to one man.
Human beings are animals with animal drives. This does not detract from the sacredness of the marriage sacrament in Christianity. In fact, it makes the analogy of Christ to the church even clearer.
Trying to understand civilization without this fact is like trying to understand physics without the second law of thermodynamics. Reproduction is the foundation of society, and so it is necessary to understand every facet of it in order to make smart prescriptions.