Gay marriage will encourage people to be gay, in the same way that hanging around tall people will make you tall.
Legalizing gay marriage will open the door to all kinds of crazy behavior. People may even wish to marry their pets because a dog has legal standing and can sign a marriage contract.
Straight marriage has been around a long time and hasn't changed at all; women are still property, blacks still can't marry whites, and divorce is still illegal.
Straight marriage will be less meaningful if gay marriage were allowed; the sanctity of Britany Spears' 55-hour just-for-fun marriage would be destroyed.