Let me say at the outset, before we start discussing anything else, that I am absolutely in favour of gay marriage – or same-sex marriage, or equal marriage, or whatever you want to call it.

If two people, of whichever persuasion, want to make a life-long commitment to each other, that’s fine by me. As a society we need more commitment, not less, and I can’t see any benefit to society in denying a couple the right to make that commitment simply because they are not of different sexes.

I should also say that I have been divorced myself, and I used adultery as my “fact”, so I know how useful it is to be able to divorce on the basis of adultery. I have also married again, which means that I am prepared to put my money where my mouth is when I express support for marriage.

Right. Now all that’s out of the way we can get to the subject under discussion, which is gay marriage. Or rather, gay divorce, since to define marriage it seems we have first to define divorce, and before we can let anyone marry we have to make sure they can get out of the marriage first. Which is a bit like making sure a room has a fire exit before entering it: a pessimistic precaution but a sensible one.

Or more precisely, gay adultery, since that seems to have become the main sticking point before we can legislate for gay marriage.

David Cameron, we understand, is absolutely determined to introduce legislation for gay marriage, despite the sage warnings of his mother, Mary. Allegedly it is his wife, Samantha, who is driving the policy. Recent revelations from the 2011 Census show that married couples now make up fewer than half of households (47%). It is difficult to understand why gay people should want access to an institution in terminal decline and even more difficult to understand why Cameron should be focusing on a minority interest rather than doing his damnedest to reverse that decline, but there you go.

Adultery for heterosexuals is defined as consensual sexual intercourse between a married person and someone of the opposite sex other than their spouse. It is organ-specific, so it involves the insertion of a penis into a vagina, and case precedent, which is important in defining those things which are not written into legislation, tells us that it only requires some degree of penetration (Dennis v Dennis [1955]).

Clearly it would not be possible to define homosexual adultery in the same way, or to have a single definition which would apply to all homosexuals.

The simplest solution to this would be to leave adultery out of legislation for homosexual divorce, after all, most heterosexual divorces use unreasonable behaviour as their fact, and homosexual infidelity could simply be classed as unreasonable. This solution fails to find favour, however, because it denies homosexuals a route to divorce which heterosexuals enjoy (oh, the irony!) and is thus declared discriminatory.

The “diva of divorce” Ayesha Vardag says, “The only appropriate and balanced way to deal with this would be to scrap the act of adultery as a basis for divorce altogether, this might indeed be the point at which adultery dies the death in order to rectify this imbalance.” So, no cookies at all unless there are enough for the whole class to have one. The only imbalance is a rather unavoidable anatomical one.

David Burrowes, a Tory MP, said rather more sensibly, “The Government have opened up a Pandora’s box”.

In our equality-and-diversity-obsessed world an “imbalance” is an abomination and so it is deemed better to rewrite heterosexual divorce and deny heterosexuals the right to commit adultery than to introduce a homosexual divorce legislation from which adultery is missing.

Taking adultery out of divorce, however, is a bit like taking the bride out of a wedding. Divorce was devised in the first place as a redress against adultery, and adultery is its raison d’être, so to speak. Unreasonable behaviour has nothing on a good, juicy, adulterous affair.

Adultery, moreover, relies on another difficult-to-define concept, that of consummation. Consummation is what makes a marriage complete, and a marriage that has not been consummated is only partial, and can therefore be annulled. If a spouse’s first sexual coupling after their marriage is with a third party, that is not adultery, and the marriage can be ended by annulment rather than by divorce on the basis of adultery.

Consummation is also defined according to precedent, and because it involved different case law the definition of sex it utilises is different from that used for adultery. Sexual intercourse in consummation cases is defined more rigidly (pun apt and intended); it must be “ordinary and complete and not partial and imperfect” (D-E v A-G [1845] 163 ER 1039); there needs to be only one incident. This means that the man must achieve a sustainable erection, there must be vaginal penetration to a significant depth and for a reasonable period of time, but there does not need to be the “emission of seed”, see W v W [1967] 1 WLR 1554. “Ordinary” was interpreted in Corbett v Corbett [1971] 2 All ER 33 to mean the organ one is born with and not (in this case) a transgendered man’s artificially constructed cavity.

This is not a pedantic discussion, though obviously most people consider themselves to be fully married once the priest or registrar says, “you may kiss the bride”, the criteria for annulment indicate that consummation is necessary. A complete gay jurisprudence, therefore, needs to include gay annulment and so must also grapple with gay consummation. This really is a Pandora’s Box.

My target is not the homosexual who wishes to consolidate commitment through marriage. Such commitment should only be encouraged as a social good. My target is the naïve minister who introduces poorly thought-through legislation, potentially damaging to society, under pressure from minority lobby groups and an ephemeral zeitgeist. The history of divorce since 1857 is full of such legislation and its consequence has always been a significant increase in divorce rates and the cheapening of marriage.  It isn’t gay marriage per se we need to fear but what the legislators may do to heterosexual marriage in the name of gay marriage.

The problem with legislating in such a vulnerable area is that other lobby groups will take advantage of the uncertainty while new laws are drafted to push forward their own preoccupations. It wouldn’t surprise me if powerful lobbyists such as Resolution and the family judiciary used the difficulty of defining adultery as an opportunity to introduce full no-fault divorce as they have been demanding for years.

Many have represented no-fault divorce as nothing less than the abolition of marriage. In her book of the same name [1] Maggie Gallagher dubbed no-fault divorce-on-demand “unilateral divorce” as it allows, with the connivance of the judiciary, either spouse to end a marriage without infraction or agreement by the other and to breach the marriage contract without liability for the costs.

It would be deeply ironic if the homosexuals, newly granted the coveted key to marriage, were to unlock the door to the chapel only to find the building beyond had already been demolished.

[1] Gallagher, M. (1996). The Abolition of Marriage: How We Destroy Lasting Love. Regnery Publishing.