So if you define marriage that way, with marriage still a fundamental right, is marriage supposed to be predicated on love or not? And do you believe that homosexuals can love the opposite sex but just choose not to? Who among us is qualified to make that determination for someone else?
If anyone is free to
cross-sex marry anyone else, that's sufficient to satisfy the right to marriage? It's too bad for them if they only love people of the same sex, and too bad for them if they're trying to raise a child with a partner of the same sex?
I'm curious, what would you think of a SSM amendment if it were passed/ratified? Would you consider it logically incorrect to define marriage that way, despite the amendment, or would you recognize a redefinition of the
social construct of marriage? If the latter, then you can't assert that marriage is between a man and a woman, because it's only that way because of cultural bias.
While it's technically true that there are means for same-sex couples to acquire (nearly) any sort of legal right with respect to children that a heterosexual parent would automatically enjoy, is it not true that it's a lot more effort and legal hoop-jumping to execute all the powers of attorneys and directives required?
Most importantly though, your position begs the question of what a marriage is. Certainly marriage is a fundamental right, and had that been denied to anyone the issue would be quite different. The issue presented is whether it violate the constitution for a state to define marriage according to gender/sex.
First principles: Marriage is a fundamental right (or so the SCOTUS has said, repeatedly). Marriage is (supposed to be) based on love. Love is subjective. State-sanctioned marriage is valued because it confers a package deal of social and legal benefits that are difficult to get otherwise.
What possible conclusion can you reach, unless you believe in some a priori definition of (additional elements of) marriage, other than: any people who love each other should be able to marry unless there's a compelling government interest in preventing it? Not a "we don't like it" or "maybe it will result in this or that bad thing" kind of justification, but a compelling justification. If the state doesn't want to be in the business of giving benefits to married couples just because they're married, it can get out of that business, leaving marriage as, essentially, a turnkey bundle of legal agreements. The State might decide to only offer certain benefits to child-rearing couples, for example. It already does so, in some ways (see later in the next paragraph).
I don't know all the legal and financial benefits of marriage, but as far as I'm aware there are some social security implications, and there are a significant number of legal decision-sharing rights that each person can make for their spouse given the right circumstances, without a bunch of extra legal paperwork. Is it such a big deal to give gay couples those things? And various other things related to households and child welfare, like child tax credits or claiming other individuals as dependents, already exist in tax code completely separate from the concept of marriage.
All the hypotheticals about consanguinity and polyamory... prohibitions against those things are not often enforced (as long as the participants are consenting adults); they're merely taboo (the former more than the latter). What we're talking about is marriage, not the underlying relationship. If there are major problems with genetic defects from inbreeding, and we want to codify a preventative social taboo by legally restricting siblings from getting married, let's also talk about forbidding people with serious heritable diseases from marrying, too. I have a feeling there would be immediate cries of, "But that's eugenics!" And, as someone pointed out, we can't use that to justify banning same-sex sibling marriages, can we?
You can very easily differentiate marriages to non-human animals, marriages to corporations, etc, on the grounds that those are not natural persons, therefore the right to marry doesn't attach to the non-human participant(s) in those hypothetical marriage applications. I think this ruling will and should put pressure on people and the courts to reevaluate the marriage of close relatives and the restrictions against multiple simultaneous marriages or group marriages.