The corollary to the argument Michael Kellahan presents (Why marriage should be between a man and a woman; SMH Aug 10 2017) is that it is ok to believe the Sun revolves around the Earth, because at one stage that is what people thought.
Just because a view was once historically popular does not mean it will always be fit-for-purpose.
On marriage equality, Penny Wong has reflected on her 2010 views and has modified them to suit the contemporary political and social climate.
It would bode people like Michael Kellahan well to adopt a similar line of thinking, because while it is ok to believe the Sun revolves around the Earth, doing so is going to make you look out of touch with the world around you.
Good point, although there is also an even shorter (one word) rebuttal to his appeal to tradition:
Slavery.