http://www.bbc.com/earth/story/20160213-why-pairing-up-for-life-is-hardly-ever-a-good-idea

Summary: Scientists at the University of Cambridge studied over 2,500 species of mammals that are monogamous and those that aren't. The difference tends to be availability of mates and how sexually dimorphic the sexes are.

Body:

The more sexually dimorphic a species is, the less likely it is to be monogamous. If males must compete to mate with multiple females, this can drive them to grow to great sizes or develop ridiculous appendages. If each male tends to settle for a single female, competition is less intense.

This explains why gibbons, which pair-bond, show very little sexual dimorphism. Gorillas show a lot: silverback males are huge compared to females. Humans fall somewhere in the middle, not far from chimpanzees.

Humans are not all that sexually dimorphic in comparison to other species. (E.g. any transgender that looks remotely passable, a Silverback could not pull that off). While we are sexually dimorphic, one could not pass off a bull moose as a cow, its simply impossible. There are enormous antlers on their heads. Essentially, where males must compete for mates, being extremely fit obviously makes you a stronger competitor (i.e. larger moose antlers or for us, shredded and attractive). Straight from the scientists, or the author citing them, we are somewhat sexually dimorphic. Making inferences from what these studies say about animals which are sexually dimorphic, we have some competition for mates to contend with. Nothing spectacular of course, but science is science.

If females are widely dispersed, as with dik-diks – or indeed Schistosoma worms – then males will make the evolutionary transition from free-spirited bachelors to doting husbands. This helps them ensure success with at least one member of the opposite sex.

Chimpanzees live in multi-male and multi-female communities, promiscuously mating with each other.

Essentially, if we were lone beings travelling in a empty forest of course it makes evolutionary sense to snatch up the first piece of tail that walks by. For Humans, availability of mates isn't the problem for 99% of us. So why aren't we more like Chimpanzees, for example? The studies indicate that we are similar in terms of sexual dimorphism. We too, live in communities with many males and females. Why aren't we having giant sex parties with no regard for monogamy?

There are two possible levels of monogamy. "Social monogamy" refers to a male and a female who are spatially close together, having sex, and cooperating in tasks like parenting. But it does not necessarily mean they are not sleeping around. Such exclusivity is called "sexual monogamy" and it is much rarer. A cynic would say that many human relationships are social monogamy masquerading as sexual monogamy.

Oh, there it is.

Conclusion: Scientists basically discover Red Pill truths in other animal species, notably those similar in terms of social arrangement and the level of sexual dimorphism within the species. They make wonderful comparisons to humans and our own mating habits. The author says that a "cynic" would take these findings and label us "socially monogamous" while masquerading as "sexually monogamous." Well shit, sounds like one of us sent these guys an email.

Tl;Dr - Get as fit as possible, because we aren't a sexually monogamous species and females have many options for mates.