Basically it boils down to the fact that a human male cannot reproduce without the enthusiastic cooperation of a human female. Since the woman does not want to be left literally holding the baby while the guy skives off to spread his seed elsewhere, she refuses sex unless and until he shows his willingness to engage in monogamy.

This factor over a million years or more has led to behavioural change via evolution. Males have evolved the ability to fall in love, and become faithful to one woman. Quite simply, for most guys, it is the most successful reproductive strategy.

Associated with that is social change, and most societies encourage monogamous relationships. This will be mainly to ensure that the pregnant woman is looked after. The monogamy will be enforced by the woman’s parents, siblings and other relatives. In the old tribal set up, this meant enormous pressure on a guy to settle down with one sex partner. Or else!

Of course, there will always be a few males who are so attractive to women that they can reproduce very successfully, thank you, by being promiscuous. For this reason, the tendency to promiscuity remains a strong male trait.

I would believe that females actually have the power in choosing their mate, which is coined as ‘sexual selection’. It is beneficial for men to be polygamous, while it is more beneficial for the female to find one suitable partner.

If it were not for our culture and morals supporting a one-party system, then men would undoubtedly strive for more partners. It is only natural. Men appear superficial and choosy, but ultimately could readily mate with any female that is alive.

We strive to acquire the perfect partner who shows multiple advantageous traits. If we find one, would it not be more beneficial to stay with one partner and have multiple children, rather then risking everything by having multiple relationships?

Human situations and interactions no longer follow any natural law. There are far more intricate pieces at play then merely "I see female, want to mate with female, me tired now, sleep…"

Actually, human relationships greatly follow natural laws.

Procreation isn’t just about spreading your seed, but ensuring that your offspring survive. Since humans only have one child per birth, rather than litters, it is important for the male to stick around and to protect that child until it can fend for itself. Also, monogamy is a sort of insurance that the offspring really does belong to the male, who will protect it. Others species are monogamous for the same reasons and also "cheat" for the same reasons (better resources – read on). In addition, women have concealed ovulation. While men respond to women differently throughout their menstrual cycles, men really cannot consciously tell when a woman is most fertile, and so a monogamous relationship benefits the man for that insurance reason mentioned above.

Men prefer certain traits for perfectly logical reasons. Wider hips and a narrow waist are obviously female characteristics, so that tells the guy that that is what he needs to go after. Studies have shown that men prefer increasingly narrower waists, even beyond that which is considered healthy. Breast size is rooted in the resource aspect. Women who can produce bigger breasts must have more energy resources, and therefore can fulfill the task of procreating superior children. Of course, there are always trade-offs for these traits – like the peacock with the colorful feathers – but "animals" don’t consider that.

The discussion is not up with the research.
Recent major separate reviews by Bernard Chapais and Jeffrey Winking show that the human pair-bond did not evolve as a consequence of a need for male provisioning.
Winking concludes that the human pair-bond functions to increase female investment in fertility.
How could pair-bonding so function?
I’ve published earlier this year a new theory of the basis of mate-guarding and pair-bonding: ‘sex-dichotomous mate-value trajectory’ — the paper is in the Journal of Aggression, Conflict and Peace Research: ‘Beyond staged retreat behind virtual ‘gender paradigm’ barricades: the rise and fall of the misrepresentation of partner-violence, and its eclipse by an understanding of mate-guarding’ (it’s in the context of explaining why domestic violence actually is perpetrated more not by men but by women).
If a human female has only promiscuity as a mating strategy, then the mate-value (‘good genes’) of the males that successively father her offspring will decrease in line with her own falling mate-value — women lose mate-value quickly with age, because their mate-value is simply ‘fertility’, not ‘good genes’.
To get round this, the female needs to tie down the male she attracts for regular sex when she is very young, so that he doesn’t father just one child.
Hence the evolution of the human pair-bond and the associated psychology of ‘love’.
Helen Fisher, the leading expert on ‘love’, has concluded that the average duration of the human pair-bond is four years, which is usually enough to ensure the impregnation of the woman to produce a second child after she has conceived, gestated, birthed and lactated the first by the same man.
It would seem that male provisioning is in the wake of the evolution of pair-bonding, not its cause; and in any case there is considerable doubt that it is particularly important for survival, given the sharing across the whole group of the proceeds of hunting in extant hunter-gatherer societies. [This is if anthropologists are to be believed. Cultural-anthropology is not a science but a political enterprise, and data showing ancestral equality should be treated with suspicion.]