DOMA and Prop 8. Both History.

[Bolty]

I do recall reading once of a societal reason why polygamy is a Bad Thing™ - and I'm referring to the commonly-practiced form of polygamy as "one man, multiple women." The problem was that wealthy men would accumulate multiple wives, leaving fewer women for the rest of the population.

In short, being poor or underprivileged would result in your inability to find partners to reproduce with. Since this is a core biological urge of all life on earth, such poor people would be even more disenfranchised with society and thus more likely to be violent and uprising. I realize I'm really talking out of my posterior by this point, but the argument was made that polygamy contributes to the preponderance of violence in some Middle Eastern societies where it is allowed (and why the thought of being showered with virgins after death is so tantalizing).

In short, polygamy destabilizes a society because it causes more unrest. The more likely it is that your average/lower end males can find someone to reproduce with, the less likely such males will become disenfranchised. If the birth ratio of males to females was more skewed to support polygamous marriages, it wouldn't be a problem. Even if it's not true (and good luck finding a study that actually evaluates that in a scientific manner), it sounds plausible.

Of course, you can extend that argument to just about anything having to do with wealth inequality (in before FIT) and stable societies.