Legal, and I can argue this on a purely legal/economic basis, morality aside.
First, it would completly void the pledge of allegiance and therefore I would start promoting people not being required to say it in school- there's that line about "Liberty and justice FOR ALL", and liberty includes things like "the right to marry". So either the pledge would have to be revised to "With liberty and justice for heterosexuals" or I shouldn't have to say it and lie in doing so.
Second, nothing says a church is mandated to perform a marriage of two gay people. Unless there's something crazy and stupid that I don't know about here. Though, the churches- even if gay marriage is legal- can/should be allowed to refuse to marry a gay couple if they want to. Some other church would undoubtedly do it instead. So there's nothing about dictating what a church must do. I would hope. If there is it should be stricken from the argument, I say.
Third, it generates more revenue. People get married, they usually have the ceremony at a church. And then they go get shitfaced and usually have some sort of party later on, too. And, ultimately, that's more people buying stuff and more people paying a sales tax to the state government.
Fourth, it voids the last line of the only part of the National Anthem that anyone knows- "O'er the land of the FREE and the home of the brave". Can't be called "land of the free" when it was arbitrarily decided that you can't marry if you are in love with another member of your own sex.
Fifth, I have seen absolutely no real arguments from the other side that are substantiated by anything. "Gay marriage destroys society!" without saying how it does in a way that an abusive heterosexual relationship doesn't. "Gays are immoral" without stating how or citing any examples to prove/indicate that there's any real moral deficiency in gay people. "Gay marriage makes kids raised by gay parents unsure and questioning of their sexuality and lead a confused and sub-standard lifestyle". I've actually seen several articles refuting this, with one of the greatest lines I've ever seen: "Maybe they should stop talking about us and start talking with us." Now, this whole paragraph isn't to say that these are the only ones that are ever thrown around, but I see them from lobby groups and people very often.
Oh, and the people who say "being gay just isn't right" have, so far, not provided any real evidence that is viable unless you already believe in the validity of the source (such as the Bible, not to bash Christians. You can cite the Bible, sure. But unless everyone present/involved already accepts that the particular interpretation you're using of the Bible is correct, your argument holds no water. Same for citing the Torah or the Qu'Ran or anything like that). Or not bothering to have a source at all. And I've heard both "animals don't do it, why should we?" and "Animals do it, and we're not animals, so we shouldn't." Well, a platypus doesn't eat cows, so why should we? Animals don't erect skyscrapers and have books, why should we? And for the second statement, animals also eat, breathe, have normal sex for almost entirely procreational reasons (barring a handful of exceptions, such as the monkeys that give a few humps as a greeting), and all that, so why should we, since we're not animals?
Paris Hilton too, of course.
