I can't think of any, which is why I've long believed that the more rights we grant to people, the better off we'll be as a society. There simply isn't much of a downside to giving rational adults the rights they want/need. The overwhelming majority of the time, the only people who care are old folks who just want things "they way they were", which is hardly a reason to do anything, really. In 40 years, gay marriage opponents will have either adapted to modern culture or their grandchildren will be mortified every time they speak, just as my generation was mortified every time we heard a parent/grandparent utter the word nigger in conversation.