From the CCC

Here's a great quote from the Catechism of the Catholic Church:

Though human reason is, strictly speaking, truly capable by its own natural power and light of attaining to a true and certain knowledge of the one personal God, who watches over and controls the world by his providence, and of the natural law written in our hearts by the Creator; yet there are many obstacles which prevent reason from the effective and fruitful use of this inborn faculty. For the truths that concern the relations between God and man wholly transcend the visible order of things, and, if they are translated into human action and influence it, they call for self-surrender and abnegation. The human mind, in its turn, is hampered in the attaining of such truths, not only by the impact of the senses and the imagination, but also by disordered appetites which are the consequences of original sin. So it happens that men in such matters easily persuade themselves that what they would not like to be true is false or at least doubtful.

The Church got it WAY right on this one.  Just look around at how many people seem incapable or unwilling to understand a reasonable argument for why they shouldn't, say, have an abortion, or engage in homosexual relations.  There are perfectly reasonable arguments against these things, but in my experience, those who do them just won't listen.  And this quote explains exactly why.