in Australia what do you think the marriage law is intended to achieve?
Presumably the same thing marriage laws are intended to do in most countries: Provide a legal foundation for a couple wishing to cohabitate and potentially start a family together. Because society recognises that there is often inequality of duties amongst a couple, and recognises there are benefits in having children raised in a cohesive family unit, it provides legal benefits, such as combining the couple's tax burden.
On a more cynical level, the easier a society makes it for people to get married and have kids, the more the population grows, and the more people are available to work and increase the society's success on an international scale.
No law can ever regulate mans heart, that is a matter that only God can do.