It is in the interest of government to make sure the family stays together while the children are being raised, and indeed afterward as well. That is why governments have been involved, so that once married, they would stay married instead of just dropping the kids and leaving. Governments did not allow for no-fault divorces until just lately, because they didn’t want abandoned children. In fact it was easier to divorce when only the Church was running things here in Utah, than after the government took over!
Once contraceptives and the urban lifestyle made children optional, the “New Morality” took over, promoting the possibility of other options. As immorality increased between genders, it also began to increase within the genders. This is then what is putting pressure on governments to relax the definition of marriage. The people are becoming immoral in increasing numbers.
I believe that as homosexual relations are given the favors given to marriage, family tax breaks and other benefits will be reduced, and other legislation and business policies passed, that will make raising a family increasingly difficult, financially and otherwise. That will make polygamy more attractive as the only possibility, because it will be easier financially and less men will be available to be a real father, leaving many women who want family unable to do so without polygamy.