In ways, marriage is more difficult to write about than abortion. There is still a consensus in this country, even among pro-choice people, that abortion is not in itself a good thing, despite the fact that thousands of women a year have one.
In contrast, arguments about extending some or all of the rights of married people to either cohabiting couples or to same-sex couples invoke important values that underpin our society, such as equality, justice and personal autonomy.
Briefly, the argument of those who would extend marriage-like rights to cohabitees and same-sex couples is that it is fundamentally unfair to deny people access to the protections and duties of marriage, simply because they live an alternative lifestyle.
The ability to make choices and choose a moral framework is central to what makes us human. At times, we may pursue personal autonomy to the extent that we neglect more communitarian values and thereby damage ourselves and the planet, but that does not take away from the fundamental need to have some level of control over our lives.
Similarly, equality and fairness are central values. In order to trump them, it must be demonstrated that there are other values that should take precedence. That is not easy to do, not least because our ideas have changed so much that it is no longer easy to appeal to a communal sense of what marriage is about. In arguing the case for traditional marriage remaining special and different, you wouldn't choose to start from here, that is, this particular cultural moment.
The Iona Institute, of which I am a patron, tackles this question in Domestic Partnerships: A Response to Recent Proposals on Civil Unions. It suggests strongly that the State intervenes in and regulates marriage for reasons that are much more complex than recognising romantic unions.
The case for giving additional rights to heterosexual cohabiting couples appears weakest. After all, they have the option of marriage. The trouble is that one partner may veto marriage. Nonetheless, on average, according to international studies, cohabitation is much more prone to break-up than marriage.
Irish research quoted by Iona says that only one in four cohabiting relationships lasts seven years or more. Should the State give marriage-like rights to relationships that exhibit little of the stability of marriage?
In the case of same-sex couples, who are often the victims of prejudice simply because of their orientation, civil partnership appears to be a way of ensuring equality, because marriage is not open to them.
Even the most conservative of people are capable at a personal level of being generous and accepting about friends and family who adopt very different values in the area of sexuality.
However, being tolerant and accepting on a personal level is not at all the same as changing public policy in a radical way. It is difficult to explain why not, given that many heterosexual people don't think too deeply about the reasons why the State privileges marriage.
Nowadays, the romantic ideal of love is the strongest paradigm of marriage. Two people commit to loving and supporting each other, at least as long as the loving feelings last. Looked at this way, gay people could fit the criteria perfectly. Sadly, the State is not particularly interested in love, even less in romance.
Most people would agree that children do best when reared by both their biological parents. Marriage was privileged because it bonded a man and a woman together in a way that maximised the chances that any children of the union would be raised by both parents.
Marriage is tightly regulated, not just because stable marriages lead to a stable society, but because marriage is fundamentally a child-centred institution. Of course marriages fail for all sorts of reasons, but not at the rate of cohabiting couples.
In our culture, the child-centred status of marriage has been seriously weakened; children are often expected to adjust to needs of adults, rather than the other way around. Extending marriage-like rights to gay people would be the final declaration that marriage is no longer about children, because by definition, gay people cannot have children without the active co-operation of heterosexuals.
Children of gay unions are usually children of a previous relationship or some form of surrogacy.
Some advocates of civil partnership declare that children are a red herring in this issue. There is a weight of social science research, some of it summarised in Iona's latest position paper, that by and large children thrive best in stable marriages.
It seems somewhat cavalier to propose to radically alter the nature of the most child-centred institution we have in favour of an adult-fulfilment model and at the same time declare that the interests of children are a red herring.
Perhaps we are at the stage as a society where we have weighed up the pros and cons of abandoning a heterosexual, child-centred vision of marriage and decided that the gains outweigh the losses. I don't think so.
We are muddling along, not wishing to hurt or exclude people and aware of difficult situations that need some redress. We are also uneasily aware that very powerful opinion-formers believe that only Neanderthal fundamentalists could possibly oppose marriage-like civil partnerships.
The Iona paper proposes a middle way, a limited form of domestic partnership for any people who find themselves in caring and dependent relationships. Sexuality and romantic love would not be the criteria, so for example, it could cover the situation of an elderly brother and sister living together.
Limited domestic partnerships would confer no automatic rights, but would entitle people to apply to court for maintenance rights in exceptional circumstances or, in the case of a claim by a bereaved partner that proper provision has not been made in a will. Domestic partnership could be offered as proof of a durable relationship for immigration purposes. Doctors could confer with a registered partner, which would end the unjust situation where long-time partners are excluded by family members.
The limited domestic partnership model would address some injustices, while retaining the primacy of traditional marriage and as such, deserves serious consideration.