Without having read 'The Marriage Act' and any associated Regulations, it must nonetheless be clear that the purpose of the Act is to provide assurances under Law to both partners, and to any progeny of the registered union, regarding property, earnings, assets and offspring in the event of death, separation or divorce, And to prescribe the Nature of the Unions which may be covered by the Act - so excluding child marriage, or underage Marriage without parental/guardian and prospective bride/groom consent, as well as excluding bigamy, polygamy, polyandry or other than male-female consensual unions (currently).

I take it that Legislation would also exist to extend relevant assurances under the Act to qualifying De-facto relationships - though I expect a court or similar proceeding would in this case be required to establish the bonafides of the relevant relationship.
This could be messy.
I also expect such or similar separate provisions would apply in some dominions in respect to non-male/female Unions (ie. gay unions or 'Marital Unions') - given that Aus does not currently have, and may never have, gay marriage provisions.

In our domain (AUS) it appears clear that there is strong support for Gay Union to be recognised, at least as equivalent to any qualifying (ie non underage, etc) De-facto relationship - though this support is by no means universal.

In our accepted national 'culture' Marriage is between a man and a woman (though either or both may in fact be bi- or homo-sexual or possibly trans-sexual - though such non-heterosexual 'Marriages' would be deemed a 'sham' by many, and not worthy of recognition as true Marriage).
As our traditional understanding of Marriage is that accepted currently by the vast majority of Australians, the question becomes whether we should willingly change our culture to accommodate the wishes, whim or fancy of a tiny minority? Is everything equal? TBC>

Cont'd>
It is no longer mandatory to profess allegiance to King (or Queen), let alone to God, but still is to Country, its Laws and its Constitution, but what of allegiance to 'culture' and to the norms and mores at the very foundation of that culture?

We are Not about to legalise child marriage, or female genital mutilation, honour-killing, child abuse, polygamy, rape or domestic violence. Marriage as currently understood in the Act is a foundational part of ours and many other cultures, and it affords assurances and stability regarding who we are and what we believe in.

Let others believe what they will, so long as they do not thrust or impose views contrary to our foundational 'culture' upon us.
The Marriage Act is essential under Law. Have a broad De-facto Union or 'Marital Union' Act if you will, but such will never, and can never define 'Marriage', not in our 'culture', and not in our true understanding of what Marriage is really all about, and upon which the future of the human race actually depends.