WHEN I was a teenager, I once plucked up courage to ask a vicar what he thought about the issue of gay relationships. After a long pause and a deep breath, he finally replied: "Well, Jeffrey my boy, I suppose we must try to keep an open mind about the whole filthy business."

My early wrestling with the "gay issue" brought me to a conclusion that has never wavered. Unless you are genuinely called to celibacy, the God-given framework for being homosexual is the same as for being heterosexual: monogamy. It has always seemed obvious to me that being in a committed relationship with someone you love and trust to share your life with is likely to maximise the health and happiness of both of you. Yes, it is hard; yes, it can go wrong; but, whether we are gay or straight, most of us know it is the best bet, and want to live that way. At the purely practical level, marriage is good for everybody.

Those are the arguments that politicians use in favour of same-sex marriage, and they are good ones. But Christian theology has deeper reasons for saying that monogamy is good. When we love one another in a fully committed way, so that the love does not depend on eros, but on faithful self-giving to the other, then marriage reflects Christ's union with the Church, and God's with his people. Our covenant with each other reflects God's own kind of covenanting love.

The fact that we are capable of loving as God loves us is the main reason why we say that we are made in God's image. For most of us, loving someone in that way - the mystery of losing ourselves in love, to find a better self in union with another - is the best inkling we get in this life of the kind of ecstatic union with God which is what heaven will be.

This covenant theology of marriage does not depend on gender or childbirth. Even in Genesis, the reason why God makes Eve is because "God saw that it was not good for man to be alone." When Paul talks about the theology of marriage, it is never with respect to childbirth. What matters is that the covenant between the couple reflects God's covenant with us. That is why the Church has always married couples even where childbirth is impossible.

Theologically, ethically, and sacramentally, there is no difference between a gay couple and a heterosexual couple who cannot have children. So, yes, same-sex marriage can be as holy and sacramental as heterosexual marriage. Yes, God is in favour of gay marriage, and so should the Church be.

But, of course, it isn't. In its reply to the Government's consultation about gay civil marriage, the Church of England's official spokesmen described same-sex marriage as a "hollowed out" version of real, heterosexual marriage. The obvious and insulting implication is that a gay marriage is empty, missing some all-important ingredient X.

Well, I would like to hear what that ingredient X is. I would like to know what is absent in my own relationship of 37 years, and in the relationships of thousands of other similar same-sex couples, which makes them "hollow" and deficient by comparison with heterosexual marriages. I have been observing all this for a long time, and I do not believe that ingredient X exists.

IRONICALLY, the Church knows more about homosexuality than most institutions. Most of the lifelong gay relationships I know are between Christians - many of them clerics. My partner and I met at theological college, where about three-quarters of the students and staff were gay (and the college was not unique in that respect).

Once the relationship began, I went to own up to the college principal, expecting to be thrown out. His response was: "Thank God for that. You're such a bloody miserable academic introvert - loving somebody will make you a better person and a better priest." They were the wisest words that I ever heard him utter. But, of course, they could not be said in public.

This is the real problem. For decades - perhaps centuries - the Church's leadership has had a public attitude to gayness, and a private one. I have yet to meet a bishop or archbishop who, in private, is unsupportive, or seriously believes that such a relationship is a sin. The only sin is in telling the truth about it. Twice I have offered my resignation, in exasperation at all the lies, only to be told: "Don't be naïve. We need honest chaps like you."

It is obvious that a number of bishops are gay, and some are, or have been, in gay relationships, yet they constantly refer to gay people as if they were somebody else. For all the fuss that was made about Bishop Gene Robinson, there are probably more gay bishops in the C of E than in the Episcopal Church in the United States. The difference is that the Americans tell the truth.

Canon Giles Fraser put this nicely in a recent article. Mostly, he said, people complain that the Church does not practise what it preaches; but, on this issue, we do not preach what we practise.

THE Church possesses a gospel for gay people, but it cannot speak it openly to those who most need to hear. It cares too much about its own institutional politics to care about this large section of God's people. It wants to keep the privileges of establishment as a Church for the whole nation, but, in order to appease its own extremists here and abroad, it demands exemptions from equality and human-rights legislation that everyone else accepts as common decency.

By opposing almost every advance that gay people have made since decriminalisation, and now by opposing same-sex civil marriage, it has turned itself into the enemy number one of gay people - despite its being one of the gayest organisations in the country.

This is a disaster for the Church's mission, its integrity, and its morale. "A lying mouth destroys the soul," Wisdom says. It is time for the truth that sets us free.

The Very Revd Dr Jeffrey John is the Dean of St Albans and the author of Permanent, Faithful, Stable: Christian same-sex marriage (DLT, new edition 2012).