It doesn't take a scientist to figure out one of the most basic principles about relationships: males and females often have very different mindsets. Any survey of humans could show this, but the principle is universal, with the "battle of the sexes" being fought over and over everywhere from colonies of marine invertebrates to you local singles bar.
The main idea:
1. Males are capable of fertilizing many females, and so it is to their advantage to seek as many mates as possible.
2. Females are limited in their reproductive capabilities and invest more time and energy into offspring, so they are more 'careful' about critical resources, including mate selection.
But, it could be argued, there is a way to make everyone happy! In polygynous systems, several females share a single male mate. This is a widespread mating system, and is even found in up to 2/3 of traditional human societies. Boy gets many girls, many girls get boy's resources, everyone wins, right?
Obviously it cannot be this simple. If polygyny is that universally advantageous, why would any animal be monogamous? There are many examples of strong male-female pair bonds in nature, most classically avians: 90% of bird species are monogamous. Gulls, with a "divorce rate" of 0.3%, put humans and their approximately 50% rate of failed marriages to shame.
So what determines whether a species is polygynous or monogamous? There are many factors, but a particularly instructive case is that of hoary marmots, (Marmota caligota).
Marmots are Northwestern mountain animals, living in alpine meadows from Idaho and Washington up to Alaska. This is not a kind environment, and scrabbling out an existence on the cold mountainsides obviously involves many resource limitations for the animals. They are known to be very territorial, and males will aggressively defend their home ranges.
Those home ranges are the key for the principle of polygyny thresholds. This is basically a measure of how "bad" a territory must get before a female decides that she'd rather be someone's second mate than have a mate all to herself. As long as territory quality is good and plentiful, monogamy will dominate, but if it gets to the point that all the single males have poor territories, a female will choose to share a territory and mate with another female, effectively switching to polygyny. In populations where there is little difference in fitness between being the 1st or 2nd (or 3rd, etc) mate, you will see polygyny more often. This means that you can have a single population of marmots in which both monogamy and polygyny occurs, with high-quality males having many mates and males with pooer territories only being able to support a single one.
That, my friends, is a polygyny threshold. Draw human analogies at your own risk/entertainment.