Omega, maybe I jumped to conclusions about your response. I still think you are completely, unabashedly, unequivocally wrong, but I won't get hung up on a couple of your less well-constructed arguments and write a formal response to your last post.
A But if marriage is (clearly among same-sex supporters) not grounded in the traditional, natural order of things and is merely conventional, then it would be just as arbitrary and open to challenge as heterosexual marriage. Any definition you would attribute to marriage would become merely subjective and arbitrary. B So why disenfranchise polygamous couples? C And even if that definition would be accepted, then there's no way you can rule out incestuous couples. D And who says dead people can't consent? Let's say an individual in a necrophilic relationship signs a contract stating that she would consent to her partner necrophilin'g her body? E And who says children can't consent? It's a simple "yes / no, I do / don't consent to having sex with / marrying you). The legal arguments don't concern me as much as the philosophical ones.
A. If I understand you correctly, what you're saying is that the concept of same-sex marriage confuses our classical concept of marriage, ripping it from tradition and custom and making it arbitrary and meaningless. This would be a valid point if we did have a traditional and nonarbitrary concept of marriage, but we don't. Originally, marriage was a process by which the wife was given to the husband by the wife's father. With modernization, civil rights, and the like, this process became a little less misogynistic and awful, but marriage as a tradition has been in a constant state of flux for thousands of years.
The way marriage is now is not traditional or significant in any way. People can make their marriages into whatever they want them to be. Married couples can be loving, exclusive, and pious, or they can be hate-filled, open, and atheistic.
A heterosexual couple can make a marriage whatever they want; tradition and the church need not be involved. It is arbitrary. Marriage is not an ancient, holy rite. All it is is two people agreeing to get married and do whatever marriage means to them. Marriage is two consenting people agreeing to be wed.
B. What is a "polygamous couple"? Regardless, there is no reason to offer a group marriage. If we look at marriage as what it is, two people consenting to be wed, then group marriages are ruled out. Social norms and our current governmental infrastructure rely on marriages' being between two people as well.
C. I am not opposed to incest in principle. But if we have arbitrary restrictions in place in heterosexual marriage, why does the introduction of homosexuality remove these arbitrary restrictions? I'm sure whatever arguments there are against incest -- the genetic, for example -- still apply.
D. I don't know, basic logic and the definition of consent?
E. See above. Children are not capable of making decisions with such extreme consequences, and as such they cannot give consent. They have never been able to give consent. This is why underage heterosexual couples cannot marry.
Your argument is similar to one I've heard thousands of times by Rick Santorum et al.: marriage is an institution, and the introduction of homosexuality will cause a chain reaction that will allow people to marry dogs, children, corpses, and toasters. That's not the case whatsoever. Marriage is nothing. It is arbitrary. It is a piece of paper. Gay marriage only removes the arbitrary restriction that you can only consent to marry people with opposite genitals.