Back a hundred–or really, even fifty on a societal level–years in the USA, marriage was well established as “a usually lifelong bond between a man and a woman who are very strictly of the same race; in which the male initiates every major aspect of the relationship, and the female is subject to him”. The definition of marriage DOES change as society changes, and conservatives are always opposed to it, but except for fringe minorities, they always end up admitting that the just changes were better in the end. Marriage isn’t the same as it was a hundred years ago. Mixed-race couples are legal now, and even acceptable. Females have significantly more right to self-determination, and while I’m not sure 100% what the orthodox opinion is on this, most Catholics I have known consider it a very good thing. And, for good or evil (evil evil very very evil), over half of marriages now end in divorce. So I just want to point out first of all, that “changing the definition of marriage” is a non-issue. The question is whether we’re changing it in an appropriate way; because there have been lots of changes in the past couple centuries, some appropriate, some very inappropriate.
Now for the actual substance of your argument, which I summarize as the following: Under many circumstances, same-sex partners have had the opportunity to have all of their “just demands” met, such as hospital visitation and et cetera, where they receive all the same benefits as legally married couples under a social system which could be accurately described as “separate but equal”; and your second major point (more in illustration than support of the general “gay marriage is bad” argument) is that children are being deprived of something deeply important when they don’t have parents of both sexes to raise them, and that it is wrong to intentionally force that situation on a child. Please correct me if I’ve in any way misrepresented you here, and this affects my response.
“Separate but equal” simply isn’t equal. Even if the legal rights are technically there–which in most situations they aren’t at any rate, the cases you named are exceptions–“domestic partnerships” are not as good as marriages. This is true for a couple of reasons. First of all, and as a Catholic this should definitely make sense to you even though you won’t agree, legal marriage has the full force of tradition behind it. Domestic partnerships are an entirely modern, contemporary construct meant to construe certain benefits to people who are implied to be officially considered unsuited to have their love really acknowledged, but who will create too much of a fuss if not compromised with. As big a fan as I am of compromise, I’m a much bigger fan of equity. Domestic partnerships, while construing some legal benefits, simply do not and never will have the same spiritual, emotional, traditional, or societal worth as do marriages. These things matter to people; it’s not a wish for exactly-the-sameness (at least not except in extreme positions), it really and truly is a wish for equality, but because of the weight of history, equality will never be possible except through relative sameness. Domestic partnerships are a bad compromise, because there’s really no good compromise on this issue. Individuals are free to recognize as valid and spiritual any marriage they desire, and to withhold the same recognition from the same. Churches are (I believe they are–if they are not, then I agree that they ought to be) also free to recognize or not recognize any marriage they choose. But if the question is one of government and political rights, then there is a question they need to ask: is “change” actually the worst thing that can happen as a result of the legalization of gay marriage, at this stage? If it is, then “affected” or not, absolutely no one is actually harmed by it, and the government, as (in theory if not in practice) the guardian of Liberty in our society, has a positively bounden duty to uphold gay marriage. If it not the worst impact, that is, if people (especially heterosexual people) are harmed by homosexual marriage, then the government has the same bounden duty not to allow it. That leads into your second point.
Are children missing something very important when they are raised in a same-sex household? Well, the fast answer is “no, not usually”. There is some disagreement of course, but most credible psychologists (by which I mean, most psychologists who have conclusions backed with actual study and evidence, and not with unprovable, theoretical Freudian psycho-babble and nonsense) agree that the majority of children in same-sex households grow up just as functionally as those in heterosexual households. What’s more, because they often have experiences with many sorts of people, they are very often even more caring, understanding, loving, and spiritual than are children raised in heterosexual households. At least, there are few credible studies which suggest that they are less of any of those traits. It all depends on parental raising styles of course, but as a rule, the children grow up happy and healthy, and are strong, moral people by the time they are adults. Of course, if you do want to go the route of Freudian mumbo-jumbo, it is perfectly possible for children in same-sex households to find a role model of the unrepresented sex; no one to my knowledge has claimed role models have to parents…
So anyway, if I missed anything, do tell me. I await your response.