Prompted to write this by Doug's post.
The wider social group has historically had an intense involvement in the regulation of human sexuality.
The justifiable reason for this is that human sexuality has (predominantly) had the consequence of reproduction - and the wider society has a major stake in the raising of children. Given that (by and large) stable relationships are the best way to raise children it seems legitimate for society to support stable relationships.
The advent of cheap, reliable and widely available contraception changes this.
Now the expression of human sexuality has no _necessary_ link with reproduction.
This puts humanity into a new place, and we are still working through all the implications. (In particular, just because the mind knows conception will not follow, the body has its own reasons, and may not ever adjust to the new situation. I suspect men find the consequences easier than women - but perhaps I'm just thinking of Julie Gianni.)
At an early stage - the 1930's - the Anglican Communion decided that contraception was not against the will of God, and the matrimonial service was adjusted to reflect this.
Now sexuality is seen as not simply about reproduction but about mutual affection and the development of relationships.
As Rowan Williams has previously argued, once you accept this, there is no argument against homosexual relationships as such - after all, even Adam was given the final choice of mate by God.
I suspect that - in a hundred years or so - we will end up with two different institutions, one which is centred on reproduction and the provision of the best possible environment for the raising of children; the other based around the mutual giving and receiving of affection. Society will regulate the first but have nothing to do with the second. (Sometimes, of course, they may overlap.)
Final thought: people who comment blithely about a 'biblical understanding of sexuality' don't seem to read the same bible as me.
And here's something I obtained earlier: