On John Corvino's information page at Independent Gay Forum, there is a link to this set of videos of a lecture he gave entitled "What's Morally Wrong with Homosexuality?" In the lecture Dr. Corvino goes through 4 major objections to homosexuality: 1) the Bible says it's wrong, 2) it's not universalizable, 3) it's harmful, and 4) it's unnatural. Dr. Corvino argues against each of these objections, but it is his final point that I found most worthwhile. He stated that he is not arguing in favor some permissive sort of moral relativism, but that the objections to homosexuality on moral grounds are not sustainable. Instead the moral judgment should not be whom or how a person loves, but whether a person loves.
Dr. Corvino's lecture is about an hour long, and then there is a period of questions and answers following. I found most of the questions difficult to hear and gave up on them. But the lecture itself is worth a look if you can find the time.