If people are opposing gay marriage just because the Bible says that it's wrong, how come they aren't trying to shut down Red Lobster for serving shellfish (Leviticus says shellfish is unclean) or trying to stop retailers from selling clothes made from more than one type of fabric?
From the perspective of Christianity, there are a couple different types of commands in the Old Testament: those pertaining to morality in general and those pertaining to the Mosaic Law that only apply to Jews. The reason why Christians are okay with eating shellfish is the same as the reason why Christianity doesn't require circumcision. Both are Jewish practices rather than universal moral truths.
And it's total nonsense, since there is no objective way of saying which belongs to which category. That's literally handpicking what you want and don't want in the Bible.
