Polygamy and incest, in terms of immediate family members, is banned across the board in American society. Neither heterosexual, nor homosexual people are permitted to enter such arrangements. As such, as a matter of equity, there's nothing bad about it.
Your definition is a little short sighted. I mean, in the Roman Catholic Church, "marriage" is defined as a sacrament between a Catholic man and woman. As such, according to the Catholic Church, all non-Catholic "married couples" are merely living in sin, and, technically, engaging in mass fornication.
What we have a problem with here is that if we start defining legal institutions based on subjective religious practices, where do we stop? Why don't we legalize polygamy, since Islam permits it? After all, what makes Christian religious practices "better" than Muslim religious practices in a country that practices "freedom of religion"?
Secular humanism, which is what our country was really founded on (where do you think "freedom of religion" comes from? The Bible? Hah!), has determined the justification of gay marriage. It's between consenting adults. It doesn't hurt anyone. And there's no rational reason to impose gender roles as a reason to discriminate. If it can be accepted that marriages can occur without children, which many heterosexual couples choose to do, then it cannot logically be determined that marriage is about having children and families.
In the end, I believe in the right of the Roman Catholic Church, for instance, to marry who they want, and, likewise, declare who is not married in their eyes. And, likewise, I believe it is the right for people to live how they want, no matter how much it pisses off the Catholic Church or Evangelical Christians or Ayatollah Khameini.
Whether or not two men or two women get married is absolutely none of your business, just as it is none of my business regarding everyone else's marriages. If that means that a brother and sister want to get married, why should I care? I don't. I'll find it as gross as any other heterosexual union. Polygamy is a whole other can of worms, mainly because such arrangements are never equal and often abusive. We're past the days when women were merely property to be acquired, which is why those arrangements are unacceptable in the Western world.
Melon