And Epicurus thought there must be a god or gods, but they had no interest in our pursuit of happiness. But none of the Greek philosophers suggested we needed the power of God to live our lives. So Paul points out that the Greek attempt to attain perfection without God by the pursuit of wisdom had failed disastrously. This was obvious to all, so he has only to delineate the declension in three steps down into the current enormities. The next step that inevitably goes with idolatry is sexual degradation.
Sex becomes the worship of the male or female body instead of a celebration of love.
What horrified Paul was that these practices that had required the death penalty were now common pastimes in the Greek world. One hates to discuss such crudities, but it seems that God's word in Leviticus and Romans is not embarrassed to call a spade a spade. And we know that these three kinds of behavior were flaunted in the world where Paul did his apostolic work. It was characterized by moral evil, greed, malice, envy, murder, strife, slander, inordinate pride, constant invention of new forms of evil, hatred of parents, implacable ruthlessness, etc. The end of civilization is when such social behavior is applauded and made the norm in society.