Read Judge Posner's original piece here (and especially the replies in the combox). And then read John Breen's response at the Mirror of Justice.
I have, for years, been in awe of Judge Posner's imposing and impressive intellect. In fact, in my philosophy of law class I require my students to read several of his articles. But on the matter of Catholic moral theology, I will pass on the prince of law and economics and defer to the wisdom of the Prince of Priests.