In one of its most famous episodes, Martin Sheen's President Bartlett reminds us that the Bible also sanctions selling a child into slavery and putting to death someone who works on the sabbath. It says touching the skin of a dead pig makes one unclean, you can be stoned to death for planting crops side by side, and kill someone for wearing garments made from two different threads. Oh, and ever so subtly, the Bible also teaches us that we cannot eat shellfish.
"Because the Bible says so" just doesn't seem to be a great answer to the question, "Why is gay marriage wrong?" now, does it?