Is it ever licit for a faithful Catholic to disagree with an authoritative, non-infallible teaching of a pope? Yes. If a person has inquired diligently into the teaching in question, and if after serious prayer and reflection, feels that fraternal correction is in order, then one may express this disagreement publicly as long as: (A) one’s reasons are serious and well-founded; (B) one’s dissent does not question or impugn the teaching authority of the Church; and (C) the nature of the dissent is not such as to give rise to scandal.
I’ve often thought that these would serve as good rules-of-thumb for disagreeing with almost anybody. You should have good reasons for your position; you should strive not to impugn the integrity or good intentions of your interlocutor; and you should argue in such a way as not to give scandal. One rarely wins over others (including bystanders) by brow-beating them; you usually succeed only in making your side look bad.