Pablo, I think you'll agree for those of us who come from Catholic (or in my case Orthodox) backgrounds there is not only a big emphasis on the biblical text itself, but on "Tradition" and "Patristics." The idea is, the Church Fathers said this and that, so we have to believe this and that. And heaven knows they were not gay-friendly. I was so disappointed to read St. John Chrysostom, who in one breath writes of mercy and grace, advocating death for those guilty of same-sex acts.
So, when talking to a traditionalist Catholic, you'll not only have to explain how their reading of the Bible is wrong, but have the added (and harder burden) of convincing them that the "infallible" Church Fathers (Basil the Great, John Chrysostom, Augustine, Cyprian of Carthage, etc.) were wrong. Sometimes Protestants refer to the fathers as well, but not as often--they pretty much stick to just the Bible. For Orthodox Eastern Christians, the words and writings of these people is basically on equal par with scripture, so we have a problem...
Plus you'll have to convince traditionalist Catholics that their infallible popes were wrong...
"And though I may not know the answers, I can finally say I am free. And if the questions led me here, then I am who I was born to be." --Susan Boyle
"If all fools could fly, the sun would be eclipsed forever." --Dutch proverb