So, Colin Craig is arguing that its alright to discriminate against gay people by denying them the right to marry because its a "tradition". But to point out the obvious, many of our "traditions" aren't that pleasant. Racism is a tradition. Sexism is a tradition. Domestic violence and the death penalty were traditions. And our society has looked at all of those traditions and decided that they are wrong and that we are better off without them. While we have a way to go on some of them, the overt, legally sanctioned discrimination and violence of those traditions has been outlawed.
It's called "progress". And it allows each of us, including Colin Craig (a member of a religious sect which would have seen him burned 500 years ago, and barred from public office 200 years ago), to live the lives we want, according to the values we choose, free from physical and social violence from others. Removing discrimination from our marriage laws is just the latest stage in this journey. And if Colin Craig wants to stop it, he's going to need a far better argument than just "it's tradition".