Take a look a how North Dakota is trying to defend their marriage discrimination laws in court:
The man-woman marriage institution has uniquely provided valuable social benefits necessary to the well-being and stability of society and the development of individuals, especially children. In particular, the man-woman marriage institution's norms and other public meanings have helped a greater portion of children know and be raised by their mother and father.
Toward the end of the twentieth century, however, various individuals and groups began a campaign to use the force of law to replace the man-woman marriage (traditional marriage) institution with an institution that would still be called "marriage" but would have a very different core meaning: the union of any two persons without regard to gender (genderless marriage). This civil action is an important part of that campaign.
[…]
North Dakota can have only one social institution denominated "marriage." It cannot simultaneously provide the historically proven valuable social benefits of man-woman marriage and the asserted benefits of the new genderless marriage. One necessarily displaces or precludes the other.
Let's break this down bit by bit:
The man-woman marriage institution has uniquely provided valuable social benefits necessary to the well-being and stability of society and the development of individuals, especially children.
Maybe so. But the issue there is whether opposite-sex couples should be allowed to marry, not whether same-sex couples shouldn't be allowed to marry.
In particular, the man-woman marriage institution's norms and other public meanings have helped a greater portion of children know and be raised by their mother and father.
Opposite-sex couples will not have to give up their children if same-sex marriage is legalized. The number of children being raised by their mother and father will not decrease.
Toward the end of the twentieth century, however, various individuals and groups began a campaign to use the force of law to replace the man-woman marriage (traditional marriage) institution with an institution that would still be called "marriage" but would have a very different core meaning: the union of any two persons without regard to gender (genderless marriage). This civil action is an important part of that campaign.
I guess that's reasonable. I would say that it's not so much getting rid of an institution and replacing it with something else, but reforming an existing institution, but we do seek to make it that gender is not a barrier to being able to marry who you love. But I am unaware of any harm that making gender irrelevant will cause.
Here's where it gets hilarious:
North Dakota can have only one social institution denominated "marriage." It cannot simultaneously provide the historically proven valuable social benefits of man-woman marriage and the asserted benefits of the new genderless marriage. One necessarily displaces or precludes the other.
Basically, if you have "genderless marriage", then you cannot have "man-woman marriage". Never mind that men will still marry women once same-sex marriage is legal. Man-woman marriage is a subtype of genderless marriage, ND. Opposite-sex marriages can take place with same-sex marriage. That happens all the time in jurisdictions where both are legal. In the 19 states that have legalized it, both gay and straight couples marry. One type of marriage does not displace or preclude the other. They happen together all the time.
Always eager to embarrass itself, NOM jumped all over North Dakota's argument. They said:
This is exactly correct. Redefining marriage to include homosexual couples isn't simply adding a parallel institution that won't alter or interact with marriage — it fundamentally changes marriage and makes it an inherently genderless institution.
Yes, I guess it's a change from the only-opposite-sex definition. But so what? What harm will it cause? It's a good thing, being able to remove gender as a barrier from love.
I'll add the "it's not possible for gay and straight couples to marry together" argument to "not all chairs are exactly the same" argument against marriage equality.