It would be a worthy addition to any entry-evel AI, psychology or cognitive science class.
The thesis (as best as I can recreate it) is that monogamy comes naturally in hunter-gatherer societies, because there isn't much stratification (wealth can't be hoarded). So a 1:1 pairing makes economic sense. Polygamy tends to happen only in more stratified societies where wealth is unevenly distributed (if you need an answer for why some men would rather be poor in absolute terms in exchange for more wealth in relative terms, look no further). Because a very wealthy man may have multiple wives, weath distribution is more even for women than it is for men.
Then there are rare societies which have institutionally enforced monogamy with lots of wealth stratification. In these societies, wealth inequality between men will inevitably lead to substantial inequality between women as well. Suddenly, the stakes are much, much higher for women, which creates a far more competitive "marriage market." This is advanced as a theory for why, in American society, women tend to dress in flashy colors with elaborate displays, while the men tend to dress like penguins (the reverse of most of the animal kingdom) - it's a reflection of where the fiercest competition is taking place.
All of this is a "just-so" explanation, and isn't scientific, but still an interesting evoluationary and economic argument.
Or it increased, I don't know. By having a family, you took care and was protected by people with similar background and interests. Dumping your wife in the past meant none would take care of your children, who were either laboring to increase home sweet home aggregated value or were your legacy, who would took your inheritance. It gave meaning to work today, because someone would use it after you're gone.
And people do some pretty hardcore stuff for love. War, even. I guess you need to justify that kind of behaviour later in your life. What good would be to sacrifice so much and forget about it next week?
Nowadays more people have their basic needs covered, and the concept of personal freedom is much more present. So you don't need a family to have a shelter or food on the table, and it's ok to do what's best for you, not for someone or something else.
What are good reads about the hunter gatherers?
But even there, ideed, the children know who their father is, even though they don't have a peculiar relationship with him.
Bonobo females use an interesting strategy that disagrees with this. When in estrus they mate with as many males in the group as possible to confuse the paternity. That way they get support from the entire troupe of males in rearing the young.