I'm a teacher, so let me explain this to you:
If you argue: Marriage is a contract between one man and one woman ordained by God
You have a weak arguement:
Marriages don't require belief in God.
Marriages don't require the use of the bible, or any reference to God.
Further, even if one is to entertain that "The bible says", one would then have to answer further questions:
If God's "plan" was 1 man and 1 woman, then incest was also part of "God's Plan."
In Genesis, one of Adam's son's has 2 wives. It isn't considered a sin. It simply states it.
Solomon not only had multiple wives, but also concubines.
His son, David, the Apple of God's eye, had 5 or 6.
In fact, the bible has a verse that says, "if a man has two wives, and loves one, but not the other..."
Word of God.
Let me draw an analogy:
If you are arguing with your parents about staying out late, you don't say:
Everyone else is doing it (because you will get the obligatory bridge jumping comparison to disqualify it)
You're not being fair (Life's not fair)
Come on! (No!)
And the conversation is a battle of ego and stamina.
Instead, you have to think like the parents:
1. I would like my curfew extended because as I grow older, I need to be given more responsiblity.
2. I have been home on time with my current curfew, so you know that you can trust me. I simply asking for another hour on the weekends.
3. I know that you care about me, so I will carry my cell and check in with you throughout the night, because I know how much you worry.
4. I know that this is hard for you, but I'm growing up, and soon won't be around you. I need to start making my own decisions. But remember - you raised me. I have your values inside me, so I just need to start putting them in motion, and testing myself a bit, a part from you. And I would really thank you if you could support me on this, because I know I will have a million questions.
These reasons answer those of the parents, not the child: they are worried, they are scared of losing their child, but they also realise that their child is becoming an adult, and with each year is a growing amount of autonomy that they need to be prepared for.
What your argument does is preach to the choir, but makes 0 convincing points to the pro-gay marriage side.
The ProGay Marriage side, however, argues with an appeal to sympathy. If your husband was hurt in an accident, is it fair for the hospital to keep you out of the ICU? Think about the hurt and worry. Now think about being told, "Only blood relatives, sorry." If you were in a coma, would you rather have your parents be able to overide your spouse on decisions of life or death? You are only alive because of machines, something you have told your spouse you would never want. You parent, not wanting to let you go, insists that the machines force you to live, although you are brain dead, while your spouse cries, knowing that legally, she can do nothing. You are a single mother with 3 boys. The insurance companies only recognize "real families" that have two parents, and so your company won't cover your kids.
These are the true moral issues that should be discussed, because those injustices are what the anti-gay marriage bills are creating - unnecessary grief for gay couples when one is in the ICU, making life and death decisions, and covering their children.
One must truly search their soul and ask themselves: Is banning gay marriage justifiable, when it creates more harm than good?
In the US, one can be a Hindu. You don't have to agree with them, but they have that freedom to practice religion as they choose. In the same way, people have the right to live their lives as they choose, whether it disagrees with us or not. That is the beauty of freedom.
And that's how debate is done.