This can be a pretty touchy subject amongst catholics so I am prepared to get called a lot of nasty names. RCs often assert a reality of moral absolutism ie that morality remains static throughout all time. I have a hard time reconciling this with the Bible, tradition, and reason. While I do not doubt that religious dogma cannot reasonably be diverse and remain true (ie either God is a Trinity or not, either Jesus is God or He isn’t etc), I don’t see the same principles applying to morality. Morality is often dictated by historical and sociological circumstances.
Case in point. We all agree (hopefully!) that slavery is gravely immoral. If I came across a slave today, I believe I would have a moral duty to deliver them from this evil and it would be seriously sinful for me to turn my back on a slave. Or even worse deliver him/her back to his captors. I believe that Harriet Tubman and the unknown collaborators of the underground railroad (en.wikipedia.org/wiki/Underground_Railroad a link for all you non-Americans) were heroes of the highest calibur.
But Paul would hardly agree with me. He did that which I believe to be repugnant in our day, send a slave back to his captor. And he asserted this was the moral thing to do. Anything else would have been privy rebellion in his view.
Another example: I believe that invading countries and killing all the men women and children of the countries that are conquered is one of the atrocious and heinous acts imaginable. Anybody who did this in our day would be monster.
But Joshua did such things during the conquest of Canaan. He is not labelled a genocidal maniac by any Christian of good will that I have met. Least of all RCs, who assert that abortion is intrinsically evil(meaning always evil, never moral). Even more problematic is that in Samuel, it is God Himself that ordered Saul to do the same thing which is supposedly is intrinsically evil.
Moving from the Bible, one could look at the example of St. John Chrysostom. He was very well known to teach and preach what we now would recognize to be anti-semitism. Something that the RCC (rightly) teaches is sinful.
So if we are to conclude that morality is absolute (it never changes) then we must conclude that Paul, Joshua, Chrysostom, and even God Himself consistently acted in way that was gravely immoral. That is a step I cannot take. I find it more reasonable to conclude that morality isn’t so cut and dry. What is immoral for us today wasn’t necessarily immoral for those who came before us. The world, and the circumstances in which we live, greatly contribute to our understanding of morality and to judge a man from the 7th century BC, 1st century AD, or the 5th cenury AD by a 21 century moral standard is erroneous.
No doubt these ideas are completely anathema to good RCs but it is a very concise thought on why somebody would drift toward the “evil” of moral relativism.