Then K Gurries at Opuscula has a look at the question using the traditional Catholic analysis of the moral act in terms of object, intention and circumstances: The sources of morality.
I found the latter article very helpful because I spent some time the other day reading Noldin (a standard Latin manual of moral theology) on this very subject, as well as H J Davis - a similar book but written in English. I was reminded of a Latin tag:
bonum ex integra causa malum ex quocumque defectuLiterally this means "good (thing) from an integral cause, bad (thing) from any defect whatever" which isn't terribly helpful. A more Ronald Knox-friendly translation into good English (used in the article Good in the Catholic Encyclopaedia) would be "An action is good when good in every respect; it is wrong when wrong in any respect."
Neither of the articles will solve the problem of the Pope's comments for you but both of them will help you with some moral theology. I certainly found them good because I am not a moral theologian; I'm a dogmatist. But you knew that already ;-)
Meanwhile, John Smeaton has been looking at the claim that "the Church has never spoken out against the use of condoms outside of marriage" and has a very useful series of quotations.