As for JDB's comments... good for you if you "support homos", which by definition includes their lifestyle. I cannot support homosexuality for religious, moral and practical reasons. There's a difference between tolerance and support, and I opt for the former. I also have no qualms about stating my personal reservations when it comes to gay marriage and to a lesser extent gay adoption.
For the same moral, religious and practical reasons alluded to I don't see it appropriate to elevate gay unions to the same level of recognition and acceptance as heterosexual unions. The purpose of marriage as I see it isn't just to crystallize the monogamous union between spouses, but to establish such towards the specific goal of procreation between these two discrete individuals... gene purity being a particularly desirous goal of the marital union. Clearly this does not apply to homosexual unions. This should suffice to establish my opposition for now, no need to explore the religious and moral reasons.
At best that is One rationalization for the role of marriage. By no means does it capture its total influence.
Another, no less significant role of marriage, IMO, is as a building block of societal order, a subunit of society. In that respect marriage and by extension family served a great purpose in the growth of large societies. In the simplest sense being responsible to and for a spouse, progeny (and/or parent/sibling) makes the larger society stronger.
The individual is less prone operate to the detriment of the general society. Even though his relationship to the wider society might be a nebulous concept and unlikely to influence his/her personal actions, the responsibility to his family “do right by people” is very tangible, especially during development.
We still see the benefits today. On average married people and stable family units have a positive effect on productivity, economy, crime, use of healthcare resources etc. None of this is diminished by the married people being of the same sex or the familial unit being non-traditional. In fact, closing it off to a section of society by offering a second-class, unappealing substitute, and limiting adoption of children lacking families, could be hurting us as a society. As an argument for the role of marriage it is no less useful or applicable than fostering gene purity.
I think you will find it hard to come up with opposition to gay marriage without referencing religion. The fact that the most vociferous and dedicated opponents are religious institutions and devotees while most secular people couldn’t care less is a good indication that the opposition is founded in religious beliefs. Practical arguments may be used to help but it does not outweigh the underlying fact that homosexuality is considered to be a sin that imperils one’s immortal soul etc.
If someone believes strongly that it is a sinful lifestyle I find it hard to believe that they are more driven by the preservation of marriage as a safe haven for gene purity and other secular rationalizations.
But once you start talking religion as a justification I can’t bite because my religion can’t be a standard for someone who does not believe the same thing.