1) that's how marriage has always been defined, so why change it now. Right, like black being slaves, and women not being able to vote? Poor logic. 2) it's a slippery slope. If we let two men get married, next will be a man and a dog. Or 3 men and 8 women. Well, that's a huge leap. I don't see how moving it from being "two people of opposite sex" to "two people" will result in us as a society lurching toward Gommorah.
We all know the above is an example of someone masking their true motives. Those are not logical answers, they are knee jerk answers people give when they have never thought about the gay marriage, they have only regurgitated what they have been told by someone (most likely clergy or someone who was influenced by some church). When you can get someone to get past those "pat" answers they will see there is no threat in equality.
Besides #1 is not even true. Marriage has never been what the religious right claims. It has never been one man and one woman marrying to produce children out of love. Thats what they claim it is, nothing more nothing less. They claim thats what it always has been. A quick google by a five year old would prove that marriage has never been that, maybe idealized as that in the last 100 years of time.
In reality marriage is a contract having nothing to do with love or children. If it was a religious ceremony as they claim the ceremony would not have "by the power vested in me by the state of ____" in the ceremony. Since when does a religion need the states power to preform a sacrament?