  Unlike my previous posts, this one is going to be more commentary instead of point-by-point. It's much easier to explain that way: In my opinion, marriage should be defined by the typical "one man, one woman" relationship. We've allowed every possible marriage that involves this theory: different races, different economic levels, etc [the only exception being consanguinity relationships]. But then, the homosexuals decided that they wanted to hop on the marriage train too. If we were to do this, we'd have to completely throw away the "one man, one woman" theory. So without a standard such as that, it would be chaos. What would stop everyone from screaming "We have a right to get married!"? Nothing. People would want to marry animals. People would want to have multiple spouses. People would want to marry inanimate objects [talk about a tax break].
And why shouldn't they be able to? After all, the "one man, one woman" standard no longer exists, so doesn't everyone have the right to get married? Well, when we have polygamy and bestiality legally permitted, that messes up everything. You have custody battles involving multiple webs of marriages, trying to sue lamps, and attempting to collect alimony payments from the horse who won't pay. The entire marriage code would almost have to be thrown out the window. Well, that's the main legal reason why not to allow homosexual marriage.
Of course, it's almost impossible to talk about this issue without discussin the religious aspect of it. However, I will save that for another time - whether or not homosexuals wed is, in my opinion, a different issue than whether or not homosexuality is wrong. Expect that topic sometime in the future. 
