Please notice that there are two logical barriers which the ecclesiastical argument cannot jump without falsifying it’s own doctrine. The first is this: The Church cannot say “Well, being that way is normal, something neutral or positive, the Church respects it and welcomes it. The Church only prohibits the acts which flow from it”. This position would lack logic in postulating intrinsically evil acts which flow from a neutral or positive being. And this would go against the principle of Catholic morals which states that acts flow from being – agere sequitur esse. The second barrier is this: the Church cannot say of the homosexual inclination that it is a desire which is in itself intrinsically evil, since to say this would be to fall into the heresy of claiming that there is some part of being human which is essentially depraved – that is, which cannot be transformed, only covered over.
Faced with these two barriers, ecclesiastical logic did a backward double-flip worthy of an Olympic gymnast so as to arrive at the following formulation: “The homosexual inclination, though not itself a sin, constitutes a tendency towards behaviour that is intrinsically evil, and must therefore be considered objectively disordered.” With this phrase, the Vatican Congregations sought to maintain the absolute prohibition of the acts without describing the desire as intrinsically evil. Nevertheless the price of this definition is very high. It obliges its defenders to insist that the homosexual inclination, independently of any acts flowing from it, is something objectively disordered. And the kind of objectivity they have in mind is deduced not from what can be known through experience, but is an a priori which depends on the Church’s teaching concerning marriage. That is to say, the a priori of the intrinsic heterosexuality of all human beings. In other words, from the presupposition of the intrinsic heterosexuality of all human beings, it is deduced that the person whose inclination is towards those of the same sex is a defective heterosexual.
Well, let us not delude ourselves here. This characterisation of the gay or lesbian person as a defective heterosexual is absolutely necessary for the maintenance of the prohibition, as the authors indicate with the “must be considered” of their phrase. The problem is that, for the characterisation to work properly within the doctrine of original sin and grace, it would have to be the case that the life of grace would lead the gay or lesbian person to become heterosexual in the degree of his or her growth in grace. That is to say, in the degree to which grace makes us more patient, faithful, generous, capable of being good Samaritans, less prisoners of anger, of rivalry and of resentment, just so would it have to change the gender of the persons towards whom we are principally attracted. The problem is that such changes do not seem to take place in a regular and trustworthy way, even amongst the United States groups which promote them with significant funds and publicity. As the senior representatives of such groups indicate: at most, and in some cases, a change in behaviour is produced, but the fundamental structures of desire continue to be towards persons of the same sex. 
This then is the conflict: for the prohibition of the acts to correspond to the true being of the person, the inclination has to be characterised as something objectively disordered. However, since the inclination doesn’t alter, unlike desires which are recognisably vicious, the gay or lesbian person would have a desire which is, in fact, intrinsically evil, an element of radical depravity in their desire. And we would have stepped outside Catholic anthropology. Or, on the other hand, the same-sex inclination is simply something that is, in which case grace will bring it to a flourishing starting from where it is, and with this we would have to work out which acts are appropriate or not, according to the circumstances, and we will have stepped outside the absolute prohibition passed on to us by tradition.
I'm still reading - James Alison's writing often takes days to get through - but it's another great one. Even just the above snippet has already, in exposing the contradictions inherent in Catholic doctrine (which is, let's face it, the product of centuries of massive intellectual tradition, and way beyond most of what we see from the Anglican side these days) started to whisper to me: "This argument is just about over, folks; they really haven't got even a thread of a leg to stand on."
The intro is truly chilling, too, and I can't wait to see how he ties it all together....