There are states and countries where marriage is two people of the same gender. So your statement is wrong.
You may want to qualify it. You may mean "my religion says it's opposite sex" or something like that.
But factually, your statement is wrong, because there are many married couples right now where the couples are NOT opposite sex.
And as Lakryte has pointed out over and over, marriage is a right. The Supreme Court has said that (see cases where prisoners have sued to be able to marry - marriage is a right). So the question is - can we deny this right to same-sex couples? is there any reason that society can forbid this right? is it going to hurt society if they get married? As even the people supporting Prop 8 admitted in the trial - SSM would be good for the kids, good for the couples, and more in line with our country's values than banning it. And these were the people who wanted to ban it!
If there is no reason to withhold a right from someone, then we need to let them exercise that right.
Let's take voting: In general, we say voting is a right. You cannot be stopped from voting. BUT - we have said that felons can't vote; that there is a benefit to society to not allowing them to vote from prison. Many states also ban ex-felons from voting. I don't agree with those restrictions on voting rights, but the courts have allowed those restrictions.
So when SSM gets to the Supreme Court, the SC will need to decide if there is some reason to withhold the right to marry from SSM couples. At that point, I think the evidence will be overwhelming that allowing SSM does no harm, and the bans nationwide will be repealed. (Similar to what happened with interracial marriage laws).