"What principles should guide us in determining this question?" asks Dr Bede Harris, senior lecturer in the CSU Faculty of Business, Justice and Behavioural Sciences.

"First and foremost is the principle of individual autonomy in relation to personal choices, which is a value which lies - or ought to lie - at the foundation of a free society. Clearly, choice of marriage partner is among the most intimate one can make, and so a gender-based prohibition on marriage is obviously incompatible with individual autonomy.

"The prohibition on same-sex marriage discriminates on grounds of sexual orientation. At its base, marriage is simply a contract licensed by the state.

“Discrimination on grounds of sexual orientation in relation to whom one may marry is therefore as offensive as would be discrimination on grounds of sexual orientation in relation to registering a transfer of land."

Dr Harris asks: what of arguments raised by opponents of marriage equality that its legalisation would cause harms or infringe other rights?

"We can dismiss the bizarre argument that by recognising same-sex marriage, the institution of heterosexual marriage would be harmed," Dr Harris said.

"A rational basis for this argument has never been presented, and it is difficult to see how it could be – unless its proponents believe that the opportunity to contract a same-sex marriage would somehow entice people away from heterosexual unions, a line of reasoning which would be valid only if one believed that sexual orientation was chosen rather than innate.

He said the second circumstance which needs to be considered is the argument that respect for freedom of religion requires business people be entitled to decline involvement in same-sex unions.

"First, the freedom of religion argument cuts both ways. Among the clauses contained in the freedom of religion provisions contained in s 116 of the Constitution is one prohibiting the establishment of any religion.

"Compliance with this requires that the state be secular in its approach to public policy. Restricting marriage to different-sex couples reflects a particular religious perspective, and thus imposes a religious view on others, contrary to s 116.

"It is therefore the prohibition of same-sex marriage, not its legalisation, which constitutes a denial of religious freedom. Leaving aside religious celebrants - who would not be compelled to marry same-sex couples under any of the Bills presented to Parliament – what of the argument that religious freedom requires that people be able to refuse services to same-sex couples?

"This argument has been raised in the United States where, drawing on racial equality cases decided during the Civil Rights era, the courts have pointed to the affront to dignity that is caused by discrimination and have upheld laws making it unlawful for businesses to discriminate on prohibited grounds such as race, gender, disability and sexual orientation.

"The courts have long held that once a business offers goods or services to the public, it steps into the public arena and breaches equality rights if it discriminates on a prohibited ground."

He said in any event, the entire religious freedom argument suffered a fatal flaw.

"Unlike in the case of a religious celebrant performing a sacrament, the sale of a cake is not a religious act. That is why non-religious marriage celebrants, cake-makers, photographers and limousine drivers ought not to be allowed to discriminate in the provision of what are simply commercial services.

"Unless a business were to offer its services only to members of a particular religion, there is no consideration of conscience which logically justifies discrimination against people on the ground of their sexual orientation.

"In Australia, anti-discrimination law prohibits discrimination on grounds of sexual orientation as well as on grounds of marital and relationship status. It also prohibits discrimination in the provision of goods, services and facilities, and so would prohibit businesses from engaging in discrimination against same-sex couples.

"This is as it should be – and this is what opponents of marriage equality are trying to reverse."