That is an elementary solution to your question.  Oh lets just marry as many whatever's that we want!  Nobody can tell me what is good for me.  Wrong.  Governments are in place, in part, to protect thier citizens...even from themselves.  Save your big brother or police state breath.I  can already feel the heat approaching from persons IMMEDIATLY looking at the extremes...

Now that we are calm.  There is an underlying reason for why most cultures have not adopted a polygamist lifestyle...do you think that religion or cultures just teach this stuff because they thought up a bunch of rules in the beginning?  Is is chance that decided that most peoples favor a monogomist lifestyle.  I think not.

No....cultures are not created by a civic methodology.  By that I mean...they don't all sit down and decide how they are going to handle situation x and y all at the beginning.  They are formed by Common methodology.  How does that figure into anything?  Lets take a few basic observances that over hundreds of years, I believe, convinced most peoples on the earth that polygamy is not in thier best interest. 

1.  Increased occurances of inbreeding and the resultant weaking of offspring over generations.

2. Competition for resources: its a hell of a lot harder to provide for a very large families when food/water becomes scarce. 

3. Competition for Sex:  and the general chaos that would result from matriachal or patriarchal successions.  without going on too much longer....Social groups might experience more in-fighting, offspring murder, and all that nasty stuff...and basically not be again, as competitive as another group who dont have those issues.

4.  its just a big pain in the ass to take care of lots of women, or lots of men all vying for your attention.

5.  There is no benefit to the group to have part of their society mating with dogs or snakes or same sex couples (ie. the reason gays have gotten the shit end of the stick for the last 10000 years or whatever.)

Modern governments are just putting these same ideas into codefied laws to prevent the same bad stuff from happening.  Its driven from the bottom up.  They are protecting us from outcomes difficult to see from the individuals perspective.  The same can be argued about drug control.  People who believe that unmitigated use of drugs is a good thing need to take a look at the effect widespread drug use can have on the efficacy of a population.  Ill give you an example to start: China and opium. 

I am not making an arguement for what is marriage and what isn't I am simply outlining the historical reasons people did not create Polygamist groups.  You are making a common assumption and are applying the 20-21st century Western assumptions of Abundant food,  huge local populations, perfectly mobile society, people operating under advance social structures, and short time spans and that's why you dont understand.   The truth is Dick, I really dont think you could handle 6 pregnant women at the same time , during the winter.....2000 years ago.  And to prevent you from becoming a burden on the community...I for one am going to step in and say. No you only get one wife.

 I am trying to attach the reasons why these ideas and social structures were built over Thousands of years...and why they persist in modern governments.  You cannot separate lessons ingrained in human communities over thousands of years just because conditions changed in the last 100.

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