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To answer your question about the why...
It has to do with the reproduction of the best species.
It has to do with the whole reason that people fall in love and have sex: to reproduce so that the human race will not become extinct.
Nature finds ways to make people (and all beings) reproduce whether they like it or not (they get his uncontrollable urge to mate, men and women) and to make the fittest reproduce (which explains for example why women like muscled man> bigger chance of survival in the wild and why men like big breast> better nutrition for baby> better baby).
To prevent the weak from reproducing, nature makes people attracted to certain qualities.
For men those are: survival skills, providing skills and protecting skills. Men that have these qualities attract women very easily, although it common day terms it's better translated to "confident men with a steady job who know what they want in life"
This is called the "alpha male", the leader of the group which all girls are attracted to because it's hard-wired in their genes that mating with the alpha male will lead to a bigger survival rate, both for the woman and the children she gets with the alpha.
And here comes the tuff part to swallow: There's always just one alpha in the group, and since he's usually mean to the other males (and the other males are not attractive), he get's ALL the woman in the group...Nature does this to spread around the "Alpha genes" which are all his positive traits that his offspring will get too (muscular build, healthy etc). In order to make sure that the good genes are spread around the globe for a overall optimal version of the species, the alpha has to spread his genes as much as possible, so that's why nature makes sure he will do this by making him, well want to have sex allot and with allot of different women.
Women on the other hand can only be monogamous for several reasons: first off a man can make every woman on his way to work pregnant theoretically, whilst a woman will have to bear in mind that if she get's pregnant, she will be off the market for 9 months, be less wanted by men afterward and will not be able to work (feed) herself and her child well.
That's why they have to stick to someone that will be there for them to whole way, That's why they HAVE to be picky.
Men are actually polygamous by nature, and most other life forms still uphold that very strongly (look at lions) but humans have developed to a point where they try to strip people off their primitive behaviour which leads to a constant struggle within men about whether they should cheat or not. And most do (believe me). They can't help it really. It's nature's fault and we just TRY to make things go our way because it seems right...So men end up having to cheat in secret etc.
Wow long speech.
Hope you understand it better now.
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