Polygamous "love"


Polygamy is legal in some countries, 
where a man can be married with more than one wife simultaneous. Just the man is allowed. 

Instead of a man cheating, can marry another one. At least, the wife knows. But is it fair?
Why would a woman agree to share her husband? 
Why would she believe that she deserves to live half a love? To have half a time? Half of attention? 

Half of everything... assuming they(wives) get the same.

I imagine, there must be some competition between the wives. 

Or the wives are not marrying for love or they were brainwashed very good. 

And, why can't a man share his wife? Why can't a woman also marry to several husbands at once? Is it promiscuous? Immoral? A dishonor? A shame?

Isn't the opposite? Why?