Yikes, it's a tricky business. People are free to believe whatever they want and I think explaining to the head of a church how they should interpret their own religious texts is asking for trouble. Personally I think it's easy to find a passage in the bible which can be interpreted as Jesus condemning gay sex as a sin, but I also understand that the specific passage I'm thinking of is a later addition to the gospels and is not in the earlier texts, but there are allusions to similar things elsewhere, but there are also gay Jews etc etc so as an atheist I should really keep my nose out of it and not tell people what to believe.
So, the problem is not one of theology but one of politics, and power, and influence. The Anglican church is, as Ian alludes to, hugely influential around the world. Ironically perhaps less so in this country than in other places. We should perhaps be asking our religious leaders to explain to people that this is what they believe but that others are not obliged to believe the same things nor to be governed by those beliefs, and that religious belief and secular governance should be kept separate.