It isn't, and should be available to all gay couples
Being gay is wrong, so they can't get married
The sanctity of marriage. No, I wasn't laughing. I was coughing. *cough* ... see?
It will set a bad example for Christian youth
I don't honestly have a good reason, but I still say no
Other (please explain)
It's the same with interracial marriage. Whites and blacks had the same access to marriage and had the same race based restrictions on marriage. It was racial discrimination.
Now, men and women have the same access to marriage and have the same gender based restrictions on marriage. It is gender discrimination. There's no way to get around this.