Their general response will be like "well, lol, those things are different, because those things are bad for society."
My question is always then, "who is to say what is "bad" for society? What does that even mean, especially outside of the Context of a God-centered absolute morality?
Also, who is to say that gay-marriage won't be a negative thing for society? How can we even measure that ?
From a Constitutional perspective, there are really two angles on this.
The two arguments are Equal Protection and Fundamental Rights.
The Fundy rights arguments has to do with what's being done, and the Equal Protection has to do with who is being "discriminated against."
Under Fundy Rights, statutes against gay sex have been overturned. Some thing Gay Marriage can be argued similarly. I disagree, because sex is something between only two consenting adults, it is quite a private thing, and privacy is a fundemental right. They do argue that choosing the type of family one has, is generally a fundamental right. Sure, that's true. Cities can't prevent a grandma from housing her two grandkids, by writing statutes that only "family" units can live in stand alone housing, for instance.
I think the argument you put forth here, falls under this fundy rights Branch, no? We don't allow just "any" group of people to call themselves a family and have government protection, do we? Here the state has a vested interest in the functioning of the building blocks of society. Scalia has made these same arguments in gay marriage type cases.
The thing to think about is whether gay marriage is more like two consenting, opposite sex adults getting married, or more like a dude having 8 wives, incest and/or bestiality.
If it's more like the former, then constitutionally, it should be allowed. If it's more like the latter situations, then constitutionally, it can be disallowed.
So far, at least when I read concurrences, the Court views homosexual marriage to be more similar to the latter.
Under equal protection, we look to several ways in which there could be a violation through discrimination. We could look to discriminatory intent, or we could look to discriminatory effect. We could also look to discriminatory enforcement of the same law.
With homosexual marriage, they would argue that there is a discriminatory effect. Here, people in similarly situated positions are being treated differently.
The question again is whether or not two consenting adults of opposite sex who want to get married are similarly situated to two consenting adults of the same sex who want to get married.
You will often see the argument made that "hey, everyone has the same right, to marry someone of the opposite sex." Although that's true, some will argue that there is precedent indicating that the crux of whether or not something is discriminatory actually comes down to whether or not the facts on which the differentiation is made on, actually effects the ability to conduct a certain activity.
For instance, we might say "hey now, taxation disproportionately harms those of Iranian heritage (since on average Persians are in a higher tax bracket than everyone else.) Is there intent of discrim? No. Is there discrim effect? Sure. Does it matter? No, because being persian has nothing to do with the effect, it's just a coincidence.
You can see that it's different when it comes to same sex marriage. The actually sexuality of the people in question is the very thing on which the discrimination is being based on.
But that raises the question again, if we allow gay marriage, then can we hold out the other types on equal protection grounds? This is what you ask with your question.
When we talk about equal protection, we have to look towards levels of interest the government has in the actions, and to what level the discrimination is suspect. If the highest level of suspect discrimination (reserved mostly for racial discrim) then the government must have an overriding interest.
If based on sex, then the government must have an important interest to be able to discriminate.
If there is no suspect classification, then the government only needs a rational basis to step in and do what they need to do to govern.
Being siblings is not a suspect classification. Government can basically do what they want here. Wanting to marry multiple people is not a suspect class, although one might argue that religion could be a suspect class. Even then the government might have an important interest in making sure that society has enough wives and husbands for one another.
On the other hand, gay marriage could be argued on the basis of sex. Now you've got intermediate scrutiny. So it should be treated differently, than your examples, under the Constitution and case law. Here the discrim is based on sex, right? Because hey, if one was a dude and one was a chick, we'd let them get married. So now the government needs a compelling interest to regulate that kind of marriage.
So, I guess that's my explanation.
Oddly enough, when I started to write this, I was in the no gay marriage camp, and after, I now think that there can be a pretty good argument for gay marriage under equal protection.
