If St Thomas More is looking down on us from Heaven, his attention will be drawn to Fr Frank Brennan, another Catholic reformer and humanist wrestling with his dual spiritual and temporal loyalties on the vexed question of what makes a legitimate marriage.
In his recent article against allowing same-sex couples to marry, Brennan describes the orthodox Catholic view of marriage, but then accepts more readily than some other clergymen that this is not how marriage is defined in Australian law.
Catholic marriage is 'an exclusive, indissoluble covenant between a man and a woman' that is 'open to procreation'. Even an infertile heterosexual union counts because it is not 'overtly' infertile like a same-sex union. I assume 'overt' means 'deliberate' or 'by design', excluding from marriage heterosexuals who contravene Catholic teaching by using contraception.
In contrast, Australian marriage law allows divorce and does not require of marrying partners the capacity or desire to conceive or raise children.
There the matter would seem to end, except that Brennan seeks a way to reconcile the two different conceptions of marriage he has given us.
He does this with an appeal to what he believes are shared cultural expectations about marriage and its relationship to the best interests of children. He states: 'every child has one biological father and one biological mother. In the best of circumstances, the child will know and be nurtured by them.'
For Brennan, it is this cultural norm which should underpin secular marriage law because 'The State has an interest in privileging group units in society which are likely to enhance the prospects that future children will continue to be born with a known biological father and a known biological mother who in the best of circumstances will be able to nurture and educate them.'
There is an obvious problem here.
Australian policy does not hold that children suffer from being raised by people who are not their biological parents. We allow children to be adopted and fostered. We allow singles and same-sex couples to have children, and to foster and adopt.
Brennan may retort that he is not talking about unfortunate situations where nuclear family units break down and others must step in to take on the parenting role. He is talking about the benefits of such units 'in the best circumstances'.
But science does not agree that being reared by one's biological parents confers any benefit. An Australian Psychological Society report published in 2007 concluded that children raised by same-sex couples are just as well emotionally, socially, intellectually and sexually adjusted as their peers. Some studies show they are better adjusted.
Brennan's ideal is a false one, and certainly not a sound basis for making laws on marriage.
Perhaps realising he is on shaky ground, Brennan provides his case with several buttresses.
The least of these is an odd unexplained reference to the possibility future children might share the genetic inheritance of two same sex parents. Needless to say, speculation about future technologies is not a good basis for law making.
Two more serious points are about human rights law and public opinion.
Brennan points out that, 'Instead of stating "All persons have the right to marry", the International Covenant on Civil and Political Rights provides: "The right of men and women of marriageable age to marry and to found a family shall be recognised."'
What he omits is that numerous superior courts from Canada through the US and Latin America to Europe and South Africa have established that the right to equality, to personal autonomy and to marry do extend to same-sex partners, and that the wording of the ICCPR is not an impediment to this.
Further, they have established a canon of human rights jurisprudence about the apartheid-level inadequacy and offensiveness of civil unions — Brennan's preferred way of recognising same-sex couples — as a substitute for equality in marriage.
As much as any Australian, Brennan knows the human rights case does not always carry the day, so he also bolsters his argument with an appeal to public opinion.
'Many same sex couples tell us their relationship is identical with marriage. Until the majority of married couples are convinced this is so, politicians would be wise not to consider undoing the distinction between marriage and civil unions.'
The first point to make is that the relevant majority here should be a majority of all adult Australians because the definition of marriage affects all who have the right to marry, even if they don't exercise that right. We know from repeated polls that a majority of Australians believe same-sex couples should be allowed to marry.
Second, if the views of married partners are called for, national polls consistently show they support same-sex marriage in equal measure to Australians more broadly. Interestingly, Australians with children are more likely to support same-sex marriages than other Australians. Clearly, Australians with children do not agree their family structure is somehow threatened by same-sex marriages.
Finally, Brennan appeals to his own experience. 'I rather like having a Mum and a Dad, and I suspect in future that will remain the case for most children.'
My first thought when I read this was how sad it is that a child should let an expectation of what a 'Mum' or a 'Dad' should be colour their appreciation of their parents. I rather like my mum and dad too, but not because they conformed to some sexist stereotype of a 'mother' and a 'father'. I love and respect my parents because of their personal capacities and values, and because they taught me to value all individuals for the quality of their character, not their race, religion or sex.
Beyond all this, the saddest aspect of Brennan's case against marriage equality is what he doesn't say about the meaning and importance of marriage.
Through terms like 'husband' and 'wife' marriage provide us with our only universally-understood language of love and commitment. Moreover, marriage is an institution which joins partners not only to each other but to each other's families and to the community.
This is why marriage creates kinship through terms like 'brother-in-law' and 'mother-in-law'. It is why wedding guests are traditionally asked to assent to the marriage. And it is why the state still has a role in registering and entitling marriages.
If and when the couple have children, as more and more same-sex couples are, marriage also includes these children in a legal and social contract that is broader than their immediate family.
In short, marriage creates inclusion and belonging.
To exclude same-sex partners and their children from marriage, or worse, to corral them with another category called 'civil union', sends exactly the opposite message. It says we cannot love as deeply, commit as strongly, contribute as valuably or parent as well as others do. It says we do not belong.
It was my hope that a reformer and humanist like Fr Frank Brennan would understand that in this materialistic world of disposable relationships, valuing love, commitment and inclusion must be our paramount goal. Instead, like St Thomas, he has lost his urge to improve society and reverted to orthodoxy when confronted with a change that troubles his Catholic conscience.
Rodney Croome is a Board Member of Australian Marriage Equality and co-author of Why v Why: Gay Marriage published by Pantera Press.