There is no particular reason why marriage should be regarded exclusively as being between a man and a women.
In addition, it has not always been the case that there were any laws that governed marriage. In Europe up until 1545, marriages were by mutual consent without the involvement of either the church or the state. It is only in 1563 that the Council of Trent decreed that a Roman Catholic marriage would be recognized only if the marriage ceremony was officiated by a priest with two witnesses. The state did not become involved in England until the Marriage Act of 1753 which required a formal ceremony. In 1837 civil marriages were recognised as an alternative to church marriages.
There is also a long history of same sex unions around the world including in ancient Greece and Rome, in certain regions of China and in ancient Europe.
In short the institute of marriage is not something that is fixed in stone. There have been many variations in the customs and laws around marriage and there is no fundamental reason why Australian law should not recognise same sex marriage. It is only because of the desire not to offend religious groups that the major Australian political parties are resistant to change.
The state should be neutral with respect to religious sentiments and should allow same sex marriage as a matter of equal right without discrimination based upon religious sentiments.