Omega: If gays are allowed to marry, then marriage becomes arbitrary and meaningless!
Everyone: But Omega, marriage is already arbitrary and meaningless.
Omega: YOU DON'T EVEN KNOW WHAT ARBITRARY MEANS I'M NOT TALKING TO YOU
Haha.
Instead of me responding that, though, a more adequate response would be:
Omega: Ok, then if marriage is arbitrary and meaningless, then what's to stop me from defining marriage as anything I (or anyone) so desire? Would anything not be able to be defined as marriage? No. Animal-Human "marriage"? Sure. Incestuous "marriage"? Sure. Necrophilic "marriage"? Sure. If there ever was a way to defeat your own reasoning...
Not really. Just because something is arbitrary doesn't mean it can mean anything and everything. People see colors differently, for example. One man's blue may be another's purple. The disparity, however, doesn't mean we can define blue as "fish". Subjectivity doesn't mean that something is all things. People have been saying this to you for several pages.
You're still working off the assumption that marriage, as it is now, is significant and nonarbitrary in some way. I'm not going to write out my initial response again, because it's a page back and ready to be read, but just answer me:
how could something nonarbitrary change as much as marriage has? If marriage is so embedded in the natural order of things as you claim, how could it be so different today than it was one, two, three thousand years ago? It becomes extremely obvious when looked through this lens that marriage is a purely social construct.
So, marriage is invented. Arbitrary. Marriage is just a formal commitment to one other person with some legal consequences. Marriage is arbitrary, but that doesn't mean marriage is anything. Animals, corpses, and children are still not involved because these are all things that cannot consent. This remains the same even if you allow homosexual marriage, because homosexuality does not negate consent.