the question of the thread is:
what are the dangers of polygamy? the question arises because there are many opinion of people that polygamy is bad. so why polygamy is bad ? arises.
It Goes against the traditional family srtucture that many people believe is a beneficial to society, but as things are rapidly shifting (for better or worse) Polygamy may very likely become legal in the not to distant future.
You ask what is wrong with polygamy, which is the popular term for polygyny, i.e. one man with many wives. The wrong is because there is no gender equality in this arrangement. One man lords it over several women, all of whom are rivals for his attention to them and their children. On the other hand, he has no male rival. He also has a series of different sexual partners, while the women have to line up and take their turn, much like cows waiting to be serviced by the bull. As well, only the first, legal wife and her children are entitled to benefit from the man’s life insurance, pensions, health, vision and dental insurance, tax benefits, etc. The remaining women are nothing more than concubines in the man’s harem, and they and their children run the risk of poverty. Polygamy (polygyny) comes from the dark ages when women had no rights whatsoever and were treated as chattels, the property of the man in the same way that he possessed sheep and goats. It is way past time this unjust, misogynistic system was kicked into the garbage can of history. Women are entitled to have equality with men. The year is 2012 AD, not 2012 BC.
Ancient Britons practiced it without it simply being about a man with many wives. As I see it, it was more about the issue of freedom; why should we belong to one another?
On the negative side, in our era one could quite easily find themselves having sex with half-sisters/daughters etc without even knowing. Though that’s more to do with random sex than communal sex.
Theres pretty simple reasons for why polygamy often has to do with one male and multiple females and it has nothing to do with sexism, though it is based in gender.
Can you guess what they are?
You only need one male to impregnate multiple females [1], so perhaps in times of war [or afterwards] when there are fewer men, then polygamy arises. Hence there would be a genetic survival strategy based on this - I’d assume.
Naturally if we were seahorses and the males carried the foetus, then the whole situation would be reversed [or at least the first part ‘1’].
1b. But one female with a load of males doesn’t confer the same advantage, though she would have a better selection to choose from. Hmm as far as I know the latter may not be true, sperms are chosen by the chemicals in the fallopian tubes.
I’d expect that the same amount of males and females all sharing would be the most efficient kind of polygamy, especially good for our modern culture, where only a couple of the females/males are required to look after the children and household. Just imagine how much cheaper your proportion of the mortgage/rent would be!
one way to beat the system 
Bingo…
I personally don’t believe i’d have much problem with a somewhat exclusive polyamourous relationship (haven’t tried it, so i can’t be sure) as long as I was the only Man involved, some may see this as a suttle form of insecurity but I don’t see it that way.
It’s simply a fertility issue, other women aren’t going to be able to impregnate each other. They can have all the sex they want and I wouldn’t get jealous (At least that’s what I think).
I think it’s a similar ethic behind saving women and children [with many others sentiments thrown in] on a sinking ship etc.
War too doesn’t have much effect on overall populations, because largely women don’t get killed.
Though with 7 billion people on the planet its probably better to let the fuckers drown or die in wars. 
…just the ugly ones. Hell they should attack each other with liposuction equipment. 
Well men are the expendable gender.
http://tvtropes.org/pmwiki/pmwiki.php/Main/MenAreTheExpendableGender
Only when there arent too many people.
From link
A feminist would complain like hell if judged by their genitals.
And from link…
Yet 90% of divorces are initiated by women, yet the men get to pay still. So breaking up families and exploiting a mans wealth regardless of how little that is, is moral? Women always play the innocent but rarely are. They tend to make many of the decisions these days, but are utterly useless at it. I could go on, perhaps I am bitter and twisted.
In old films they were always black lol, I guess being male is in a sense being black. Yet males are still supposed to be the strong ones, I guess being dispensable makes you strong. I never quite got altruism for archaic cultural reasoning, what does dying get you!
