In Australia, same-sex marriage activists are upset that polyamorists are trying to jump on their equality bandwagon. From The Australian:
THE main lobby group promoting gay marriage yesterday distanced itself from polyamorists demanding to be included in the proposed reforms, saying marriage involving more than two people would undermine a traditional institution.
As reported yesterday by The Australian, Greens senator Sarah Hanson-Young has come under attack from polyamorists, including some who are members of her party, for insisting that marriage should be between two people of any sex, but no more than two.
Greens volunteer and polyamorist Naomi Bicheno wrote on Senator Hanson-Young's Facebook page: "I am disappointed that it has come across as an official party policy position of against polyamory instead of simply pro marriage equality."
Another person posting on Senator Hanson-Young's page, Chris Atlee, wrote: "If you support marriage equality shouldn't that include supporting polyamorous marriage as well? After all, it doesn't affect any of us does it?"
Here’s the problem: There’s no principled way to exclude polyamory from marriage, if one adopts the principles being argued by same-sex marriage advocates.
They argue: Marriage is about making a contract with the person you love.
They argue: Marriage equality (the right to marry whom you want) is a civil rights issue.
They argue: Other marriage constructions will not affect your marriage.
All of these arguments support polyamory equally as well as same-sex marriage. So now, when it suits them, they’re calling on the authority of “tradition,” without any means of justifying their preference for two people only.
There is, in fact, no principle behind their preferred boundaries at all. All they can do is declare their definition to be the correct one:
Alex Greenwich, the national convener of Australian Marriage Equality, told The Australian that his lobby group's concept of marriage was "what it's always been" of "two people who rely on each other in a relationship to the exclusion of all others".
That actually is not the concept of marriage that “has always been.” The concept of marriage that “has always been” is one where the boundaries are principled because they’re conformed to the nature of reality—the complementary differences between men and women.
The reproductive system is the only bodily system that requires another person to complete it. The bringing together of two physically complementary persons completes this system, and that is the type of union that society has an interest in protecting because that act is the act that produces children (whether or not it does so in any particular case*). If the union of a man and a woman didn't have the social consequences of creating a family by nature, marriage (the stabilization of that union by society) would never have existed.
Why define marriage as two people? Because two people complete the union that society has an interest in protecting.
Why define marriage as a man and a woman? Because those are the complementary persons whose union creates new life.
The traditional marriage advocate is arguing not from bigotry or even from tradition, but from principles of reality that remain unchanged despite anyone’s personal preferences.
By contrast, these same-sex marriage advocates are refusing to apply their principles equally to everybody. This is inconsistency at best and bigotry at worst.
*The individual cases may vary in terms of the production of actual children, but it's not the government's job to ensure that marriage does lead to children, it's only its job to protect it because it's the kind of union that leads to children, and therefore needs to be protected because of what naturally will result from it in general.