Hi Alex,
Firstly, thanks for bringing this essay to my attention. I found it very interesting, and it's good to know that we have an Archbishop who is not averse to philosophy!
As regards Williams' use of Nagel, I wouldn't say that he "consistently quotes" him, but merely uses one point Nagel makes about sexual desire. Also, this point is used to advance a general theory of human sexuality, and not specifically homosexuality. So, I think your post heading here is a bit 'tabloid'! (You have a career waiting you in Fleet Street should you ever feel inclined!).
However, I think Williams does make an interesting point about homosexuality: firstly, since it is 'non-functional' love (i.e. not for making babies), then it should be seen as on a par with non-procreational heterosexual love; secondly, since being heterosexual (or celibate) is no guarantee that you are living right in the eyes of God, because sexuality is about more than just making sure your actions are legal. This last point is very important, and this is what Williams highlights as important about homosexuality: because there is no other purpose to it (no natural procreative function) then it questions the very nature of what love and desire are - this, Williams argues, is why people often feel threatened by it.
I'm not sure it's fair to say that Williams thinks certain heterosexual attitudes are perverse, or that homosexuality is an ideal, but I take your point. These are the sort of points that a tabloid newspaper might seize on and distort his message, but there is almost an inversion of the norm here. However, I think it is just used as a means of highlighting the central concern. Williams' (and Nagel's) point about mutuality is the central message: it is that which should define what is right in sexual morality, not what is merely legal or according to the literal message of scripture (whatever that might be interpreted to be).
Lastly, I'm not sure how you see Williams relationship to Nagel's philosophy of perception as problematic - perhaps you could expand on this point? I haven't read the particular essay that Williams quotes from, but do you think that he does so selectively?