What are some of the arguments AGAINST same sex marriage?

We're about to have a vote in Australia about whether we should allow same sex couples to marry.I feel like this is an obvious no-brainer. Gay people should have the same rights that straight people have and this should have happened years ago!However, there are still many people who believe that gay people should NOT be allowed to marry.I don't want to argue, I just want to be educated. Why would someone believe that gay people should not be allowed to marry?