Well they may be surprised. Firstly, we do have a clear separation of Church and State. Many people seem to forget this - even though much of the law in Commonwealth countries such as New Zealand and Australia is based upon Judeo-Christian values.
In the seven laws of Noah, which are the laws that should be subscribed by all people (not just Jews or Christians), one of the founding requirements is to establish courts of law and seats of government. As such, the responsibility lies with the people to determine the rules which govern them.
There is also a law on sexual morality, although it is only commentary which states whether or not this may include homosexuality. Definitely it includes adultery, but aren't we discussing marriage? Marriage equality surely applies adultery evenly to all marriages. You will also notice that Chabad's definition does not include Homosexual relationships.
Under religious law, we place our trust in the secular government to have the wisdom. In this case it is clear, Marriage Equality has the will of the people.
The Act allows for all legal covering of the definition of marriage by law. It also allows for freedom of religious expression, such that no congregation be required to conduct the ceremonies, if they are not appropriate. Maybe this will be tested in the future. I hope not, as it caters nicely for pluralism, in my mind.
Other arguments on raising of children etc have been covered well in other places. No need for me to cover it here, except that in 20-50 years time, our children and grandchildren may decide the laws need further requirement. Who knows.
One thing that is clear in Australia. They will definitely follow suit at some stage. Already New Zealand is engaging on marriage tourism. I predict this will come from a Liberal government. Mainly because the current Labor government looks set to be ousted in September. The Greens look to introduce a bill this May. Even if the Liberals agree in their party room to a conscience vote, it will take a while before the Act will pass. More likely, is that the Libs will block it until after the election. The irony here is the constant attacks on Tony Abbott - even though his sister is in a Lesbian relationship. Lets see if my prediction holds suit. I'd even cheekily suggest that supporters of Marriage Equality should vote Liberal as their only chance to achieve it :-)
[UPDATE] - I may be incorrect. It appears that a commentary in the Midrash explores this actual issue:
"Rabbi Huna said in the name of Rabbi Joseph, 'The generation of the Flood was not wiped out until they wrote marriage documents for the union of a man to a male or to an animal.'"So if Rav Huna is correct, then the world could actually be about to end (although there were no Jews in Noah's time). Anyway, as always, consult your Halachic authority if that is relevant to your non-Halachic secular same gender Union.