Well, multiple women (though sometimes those 'women' include some men) is the ideal marriage in most societies. The disadvantage of polygamy is the resultant gender imbalance: If you have three wives (the average in much of our species' history, gauging by human sexual dimorphism), two men will have none.
Polygamy has its good side: In many societies, a monogamous marriage is associated with greater male dominance. And polyandry? Even worse. But as much as men sacrifice this power, they gain social standing.
Also, and I'm sure Jana knows this, marriage has never necessarily meant a sexual monopoly, though that was usually the ideal.