I rise to speak in support of the marriage equality bill before the parliament. Consider for a moment a world which is much like our own, except for one major difference: marriage is defined as the union of two people with blue eyes, to the exclusion of all others. If you have the misfortune to be born with brown, green or any other colour then you are prevented from marrying another person, no matter what their eye colour. The law permits no exemptions.

Suppose that the disciples of this blue-only arrangement furnish their argument by pointing to long-standing legal precedents: 'This is the way that we have effectively always done it. This is a social basis upon which our society is structured. Marriage is between two blue-eyed people. There is nothing that effectively can be done about it.' I think we would scorn a proposition like that because it discriminates against people for no good reason. It is a law that punishes innate biological characteristics and falsely links eye colour with moral character. Furthermore, it enters the state into a realm where it has no business.

Excusing this simplification that I use by way of example, what are the practical differences between the exclusion of non-blues from marriage and the continued prescription of gay and lesbian people? Both lean heavily on the brittle precedent of history. Proponents of heterosexual marriage point to the scriptures to embellish their argument, but such a strategy does wither under the glare of scrutiny. Enlightened societies look to the impartial precepts of science and reason to make laws and decide what is right. Science, logic and philosophical reason are the ammunition of our political battles.

I believe it is wrong to deny same-sex couples the right to marriage. The extension of that right is not only morally defensible but it is also socially responsible. We should encourage any attempt to broaden the scope of marriage, because it is an institution which strengthens the bonds of our mutual obligations and promotes values we like to see upheld in our society. It is obvious that stable, committed, monogamous relationships do exist outside of marriage. Nevertheless, marriage occupies a unique place within our society. It is known as a commitment device. It legally joins a couple, while also binding families and communities. On average, those who are married are happier, healthier and more prosperous than their unmarried counterparts. To deny same-sex couples access to those numerous social and psychological benefits, as well as the substantial financial inducements to marriage, misses an opportunity to strengthen a public good.

I would also like to examine the economic dimensions of this debate in greater depth. As women have gained greater access to the job market and greater control over their own fertility, the opportunity cost of having a spouse stay at home, out of public life, has dramatically increased. Marriage relies less and less on the traditional breadwinner-homemaker dichotomy. Instead, we are now more likely to search for a soul mate with whom we share interests, preferences and beliefs.

Modern marriage is no longer the handmaiden of biological caprice. It is no coincidence, so writes Justin Wolfers for Bloomberg, that many of the opponents of same-sex marriage are also opponents of the ongoing shift to marriages of equality. Individuals, especially women, will continue to command greater wages outside of the home. The cost of staying at home will continue to rise.

Research has yielded evidence that there would be very real economic benefits to South Australia adopting marriage equality. Professor Lee Badgett of the University of Massachusetts has estimated that if a state like South Australia became the first state to allow same-sex marriage, its economy could benefit to the tune of at least $96 million, with most of this going to small business. I think we are very much aware of the economic benefits of attracting the 'gay dollar'.

There was also a substantial body of work done by Richard Florida who looked at the most vibrant cities around the world and was able to draw a direct link or correlation between the prosperity of those cities and the fact that they had very high concentrations of gay people. So, this proposition is not to be dismissed.

In this way, one clear argument in favour of this bill is a utilitarian one: extending the right to marry to same-sex couples will yield substantial social gains. It will strengthen our economy, and offer all same-sex couples the right to choose whether marriage is the appropriate expression of their love while the institution—although it is under a bit of stress—is given a firm jolt in the right direction, as far as I am concerned.

Now I would like to turn my attention to the legal and moral deficiencies of the status quo. John Stuart Mill, the 19th century philosopher of English libertarianism—and this should be of interest to the other side of the house—advanced a crucible of contemporary political thought: the harm principle. This idea effectively states that individuals should not be constrained from the pursuit of any endeavour they choose, up to the point of inflicting harm on others.

Failing to extend the right to marriage denies same-sex couples the chance to formalise their commitment to one another and renders them second-class citizens before the law. It signals that the political community deems an entire class of relationship unworthy of the respect and recognition afforded to heterosexual marriage. In doing so, it inflicts an unwarranted and indefensible harm.

The liberal project commenced by John Stuart Mill aspires to the construction of a state which fights for the needs of the deprived and, where applicable, does not interfere in the wishes of individual. As long as there is injustice which can be alleviated through political means, then there is a need for a strong state and intervention. That does not mean that an inequity levied by the state itself should be allowed to stand; on the contrary, it becomes incumbent on political actors to correct for obvious defects of the law.

Throughout history, each generation has expanded the freedoms won by their parents and grandparents. The extension of rights to the greatest number is animated by that same ideal. Our political process has allowed us to work at a steady pace to remove the impediments to the realisation of the equality of all men and women. Opposition to marriage equality is the next great barrier which we must collectively hurdle.

The shadow attorney-general has justified his party's refusal to support marriage equality by explaining that 'marriage is the domain of the federal parliament, so our party'—the Liberal opposition—'will not be supporting laws we don't think the parliament has the power to make'. This is a misreading of the interaction of state and federal laws, and I know it has been dealt with at length by speakers on the other side of the house.

It is true that where there is an inconsistency in a concurrently administered law, the commonwealth prevails to the extent of the inconsistency as per section 109 of the constitution, but this is no barrier to this parliament passing the marriage equality bill. In order for this bill to be inconsistent with the commonwealth Marriage Act within the meaning of section 109, it would need to purport to do the same thing. Whether that means that is unconstitutional or inoperable is a judgement which will ultimately fall to the High Court in the event of a challenge. The court would then determine the scope of the commonwealth Marriage Act, whether it comes before and at the expense of any state marriage laws or whether it can exist alongside the state laws.

In the words of Professor George Williams, the constitutional law expert who advised the drafting of this bill, South Australia would be 'doing a great service to the nation' by becoming the first state in Australia to pass such legislation and subjecting the federal act to legal scrutiny. Our parliament has the authority to pass the marriage equality bill. Objecting to the passage of this bill by taking refuge behind jurisprudence which is yet to be created dodges our duty to debate social issues on behalf of South Australians. If we value this chamber, the ideas which it embodies and the people it represents, then I urge my colleagues in this chamber and on the other side of the house in particular to not hide behind such a thin facade.