Many Christians argue that it is.
To clarify the issues the question is divided into three:
is it condemned by Christianity in general;
or more specifically by the Anglican Communion;
or more specifically still by the Church of England?
1) Does Christianity condemn it?
Some Christians argue that it is impossible to be a Christian unless you disapprove of it. Usually they base their arguments on biblical texts or the history of Christian ethical teaching. To summarise, Christian leaders have in the past usually seen it as sinful, but not as sinful as many other things we don't condemn today - like contraception, non-penetrative sex within marriage, and divorce - and never before the later twentieth century have they treated it as more important than other ethical issues.
2) Does the Anglican Communion condemn it?
Many supporters of the Anglican Covenant have argued that the Anglican Communion is committed to opposing it. The arguments appeal either to the biblical texts or to the 1998 Lambeth Conference's Resolution 1.10, where the relevant paragraph states the Conference
while rejecting homosexual practice as incompatible with Scripture, calls on all our people to minister pastorally and sensitively to all irrespective of sexual orientation and to condemn irrational fear of homosexuals, violence within marriage and any trivialisation and commercialisation of sex.
It is those three words 'incompatible with Scripture' on which the argument is based. The Resolution was indeed passed by the Lambeth Conference, but not in happy circumstances. A behind-the scenes initiative had been planned to embarrass the conference organisers and replace a more moderate resolution with a more hardline one. Njongonkulu Ndungane, Archbishop of Cape Town, and John Shelby Spong describe what happened.
Nevertheless the Resolution as we have it was passed. It is often treated as though it settled the matter once and for all as far as Anglicans are concerned. However it does not. Lambeth Conferences do not legislate for Anglicanism: they are conferences, not decision-making bodies. They have in the past expressed strong views on moral issues, but they have also changed their minds on them.
3) Does the Church of England condemn it?
In addition to the arguments above, some appeal to two Church of England publications. Issues in Human Sexuality is a slim booklet, published in 1991. It argues that the conscientious decision of those who enter homosexual relationships must be respected, but with the exception of clergy - who may not do so because of 'the distinctive nature of their calling, status and consecration'. It describes itself as a discussion document, taking the trouble to deny that it was the last word on the matter. Eleven years later, when Rowan Williams' appointment as Archbishop of Canterbury was announced, and in the following year in the debates about Jeffrey John and Gene Robinson, it was widely cited as the Church's teaching.
Some Issues in Human Sexuality is a more substantial book on gay and lesbian sexuality, published in 2003. Although it claims to offer a balanced account of the biblical texts and recent research on the causes and effects of gay and lesbian orientation, most independent commentators consider it heavily biased against same-sex activity. For a better account of the scientific research see our Forewords publication: Created by God: Christianity and Homosexuality in the 21st Century (free to download).