'The easiest moral question we've ever had to face'

Earlier this week I found enough free-time to watch one of my favorite shows, The Daily Show with Jon Stewart. The episode concluded as always with a one-on-one interview with a guest. This night's interviewee was Sam Harris author of The Moral Landscape: How Science Can Determine Human Values. It's a very compelling interview. However, a very small portion is relevant to class. When discussing why he believes science can answer moral questions as well as, if not better than, religion Harris says, "The God of Abraham gets slavery wrong. Slavery is probably the easiest moral question we've ever had to face...[Scripture] doesn't get that right."
Not being familiar with the scriptures that he is talking about, I have to take Harris' word, whose job it is to study and write about religion, as truth. While most everyone would agree that slavery is immoral, this quote made me think of a more modern debate. In my mind the same thing can be said about the interpretation of the Bible that condemns gay marriage. Those who use the Bible to support the movement to ban gay marriage need only look at the example of slavery to see that conventional wisdom can be greater than religious doctrine in moral debate.
The daunting and complicated question these examples pose, and which I now pose to you, is when scripture is morally wrong, where do we turn for moral guidance? What, if any, other guiding principles do we share to tell us when something is wrong or right? Harris would argue that the answer is science but I don't fully buy that. Then again I pretty much undecided on all aspects of this question. Slavery may have been the easiest moral question; however, it begets one of the tougher ones.