People don't understand that equality of rights isn't de facto equality. Examples:
A. Equality of rights: Men can marry women, they can't marry other men.
De facto inequality: Heterosexual men can marry whomever they love, gay men can't.
B. Equality of rights: Everyone can travel.
De facto inequality: Poor people don't have the means to travel.
C. Equality of rights: Everyone can run in the Presidential election.
De facto inequality: Everyone isn't called Bush.
D. Equality of rights: Gospel is forbidden.
De facto inequality: Gospel lovers gospel can't listen to their favorite music.
F. Inequality of rights: Black men are not allowed to enter buses.
Conclusion: It's up to you to decide what kind of society you want. If you prefer a society in which same-sex marriage is the standard, then it's perfectly fine as long as you can reasonably support your opinion. But saying that gay people shouldn't get married because the Bible said that this is bad is certainly not an admissible opinion.
Now people usually dismiss the relation between marriage and civilization.
Civilization: Eating with cultery (Europe), chopsticks (Asia) or bread (Middle East).
Barbarism: Eating with your fingers, e.g. fast food.
Now regarding sexuality:
Civilization: Rules regarding sexuality.
Barbarism: Free sex.
We are now entering a new era in which homosexuality is seen as the exact equivalent of heterosexuality. Does that mean that there is no rules anymore and that everyone will be equal? No. Examples:
A. Polygamy, polyandry and other multiple-adults families (e.g. trouples, jedi marriages) are not allowed.
B. Pedophilia isn't allowed.
C. Zoophilia isn't very well tolerated either and people can't get married to animals.
In all of those cases, we have de facto inequality. So if you believe that gay people should have the right to get married because de facto inequality is bad, then you will have to allow marriage for at least A and C. Of course you wouldn't allow marriage for B, because you consider that B is harmful to society, which is right. But de facto inequality will remain. In A and B, there is no reason to forbid marriage, but people are still not ready for that. It's very well possible that in a few years we will allow marriage for A and B, who knows?
In conclusion, the only good reason to support gay marriage is that many gay people want it (although not all of them), and an important part of society is indifferent or supportive. But there is nothing rational there, it's just a cultural evolution. So in terms of overall happiness it's better to allow gay marriage, unless we can prove that gay marriage is harmful to society and that the damages that it could provoke would be more important than the happiness that it would provide. Good luck to find the answer to this.