Much like the issue of abortion, the issue of gay marriage can be almost completely summed up in one question. How does our culture, as a whole, define marriage?
See, marriage may be a lot of things, but it isn't a scientific issue. It isn't an intrinsic right, nor is it a necessity. One can live without marriage.
Marriage is, however, cultural. It is abstract. It means only what the culture in which it takes place says it means. Therefore, it is up to those who make up the culture, namely, the members of society, to decide what marriage is to us.
In a Democracy, the government is obliged to act as a servant to society. It cannot run the country how it wishes, but instead must run the country how society wishes. It's only other obligation is the rights of minorities within society, which must be protected whenever societies wishes to trample those rights.
Marriage, however, cannot be affected by the issue of minority rights, because, as mentioned before, it is solely a cultural institution. Secularly speaking, it is not remotely necessary to one's existence, nor can official recognition of a marriage be considered essential to one's life, liberty, pursuit of happiness, or property.
Therefore, the government must define marriage as the majority of society defines it. If the majority of society decides that a person can marry his or her dog, than the government is obliged to accept that defintion of marriage. Likewise, if society decides that marriage is solely between a man and a woman, the government is obliged to accept.
In a representative Democracy, such as the US, the legislators act as the voice of the people. They represent society. Thus, it is up to the legislative branch to decide how marriage is defined. Alternately, a popular vote on the issue would be more accurate, and could also fairly determine the definition of marriage. But because it is not in the Courts duties to legislate, and society's definition of marriage does not interfere with one's rights as defined by the Constitution, both in writing and in spirit, it would be unjust for the Courts to overturn any decisions regarding the matter.
The reasons for society's definition of marriage are irrelevant. No breach in the separation of Church and State could occur when creating a law to match society's definition of marriage, as the State would not be endorsing any of the reasons for the definition; only the definition itself is taken into account. In a similar manner, it does not breach the separation of Church and State when murder is outlawed, even though many people are opposed to murder for religious reasons.
So there you have it. The question is simple. How do we, as a society, define marriage?
I see marriage as an eternal union between a man and a woman. I would like to see a law that limits it as such, but I don't expect the majority of society to agree with me. Like abortion, the main question, in an individual sense, comes down to personal opinion, and nothing more. No amount of arguing can change that, as most of the opinions cannot be logically refuted or proved. Therefore, we must take whichever opinion the majority has.
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But wait! I know your knee-jerk reactions better than you do. So here are you objections, and my responses. This way we can get a head start.
"1) Marriage isn't purely cultural, dude! There is government influence, thus giving it ecnomic and legal aspects as well."
This is a legitimate complaint, and one that forced me to rethink my proposal hard. Basically, the government would either have to get out of marriage altogether, or offer an equal status of some kind. Or, perhaps, offer the benefits in a way other than a marriage or union. Of course, with a homosexual union, all benefits that relate to the possibility of having children are unnecessary anyway.
There are also arguments that render this completely unnecessary because they oppose the original objection itself, but I needn't get into them now. Especially because I haven't been able to articulate them, so I don't know how solid they are.
"2) You're discriminating against homosexuals by making them second class citizens, and thus it would be unconstitutional to do this."
This one is not legitimate. A law that defines marriage does nothing except define marriage. It discriminates not against people, but against things within the institution it seeks to define. Some peoples are excluded by the nature of the instituion, but this cannot be equated to making the peoples second class citizens.
In the same manner, persons with certain conditions are excluded from the military. This is not because they are second class, but it is because they are naturally incompatible with the nature of the military. The exclusion of females from the draft may, perhaps, be the same case, although I hesitate to use that as an example since there is some degree of controversy surrounding the issue.
To sum it up, we're looking to define the institution, not define the people.
"3) This violates the separation of Church and State."
Also not legitimate. I think I already addressed it, though, and I don't need to repeat myself.
Skip over the typos. It was late and I couldn't sleep. That's why I'm here, after all.