Two decades ago — and for all recorded history, for that matter — "marriage" was defined as a relationship between people of opposite gender. (Certainly, many societies permitted (or even glorified) same-sex relationships, but they didn't seek to define them as "marriage." That's entirely modern.)
Today, the desire is to define "marriage" as a relationship between two people:
Marriage is the process by which two people who love each other make their relationship public, official, and permanent. It is the joining of two people in a bond... [Psychology Today]
So, what's the big deal? If two people love each other, shouldn't they be allowed the same rights, privileges and responsibilities, no matter their genders? [link]
Marriage is a ritual or ceremony which legally binds two people together. Marriage can be achieved through judicial means of applying for a license, as well as religiously through a clerics consent to espouse. After marriage, two people are joined together in a legal, social and often religious contract... [link]
But, wait, what happened to polygamy? In the US, polygamy was always believed to be a form of marriage (nobody ever said it "wasn't marriage"), but just one which wasn't legally recognized, nor encouraged. Oddly, now we are claiming, under the pretended new definition, that polygamy isn't even a form of marriage.
If we really believed this to be the definition of 'marriage', we could not say that Mohammad had multiple wives. Those who purport to accept the new definition of marriage must say that Mohammad had one wife (again, marriage is a relationship between two people) and simply slept with the other women. (Now try saying this in Saudi Arabia!)
Obviously, nobody speaks this way about polygamy — which means that everyone — including promoters of same-sex marriage — actually believe that the term "marriage" includes polygamy. They just refuse to grant polygamists those marriages same legal standing they want for same-sex couples.
In contrast, a Christian (including myself) might say that marriage is a bond between people of opposite sex (as all people in history have believed), and also argue God'sideal for marriage was one man, one woman, as long as both live. This doesn't mean that I won't call two divorced people "married." Failing to be an ideal or perfect X doesn't mean you're not X at all. A polygamous marriage (or a third marriage) is still a marriage. A flawed human being is still a human being.
One solution to this is to fall back to admitting other definitions of marriage (and scrapping their current one), but only wanting the two-person part to be legal. But how is that defended? The Christian defends the two-person part by citing Jesus — who also, annoying, defined them as being of opposite gender. So the same-sex marriage advocate can't use that, nor the Jewish (nor Muslim, nor Hindu) understanding of marriage.
On what grounds, then, to exclude Muslims, and other polygamists? On the grounds of "love"? What if one man truly loves two women, or at least claims he does? (Or all three love each other?) How do you prove such a thing can't happen?
Or do we fall back on the "it's not good for society" argument? Based on what research? Same-sex marriage activists have spent a long time explaining that marriage is about individual needs, not society — although I suppose one can always do an about-face.
I can see no rational basis for both throwing out the all-of-history definition of marriage as being about joining two different sexes, while simultaneously all forms of polygamy from fitting the definition of marriage, much less legal sanction.
Of course we can always fall back on unevaluated hypocrisy and contradiction. That's sustainable, at least for a while.