Re: Utilitarianism vs. common good
Most modern Catholic bishops (in fact, most men of religious authority) are not particularly philosophically literate.
In any case, Utilitarianism is not simply about the 'greatest good for the greatest number of people' but is ultimately about the maximising utility (or 'the good') and minimising negative utility (or 'the bad') as summed among all morally relevant beings. In classical utilitarianism, 'the good' is happiness or pleasure and 'the bad' is pain or suffering.
According to that definition, I think it is quite easy to understand why many Roman Catholic moral philosophers reject classical utilitarianism, although there are other forms of utilitarianism and consquentialism.