One reason is  fear and anger from people who see Civil and Human Rights as some kind of finite good like a pie. They're afraid that if someone else gets some, they'll somehow get less. Which of course makes  no sense at all. 

 Another problem is the misinterpretation of the Bible by generations of Christians and a mule-headed unwillingness to rethink anything based upon modern science. For example, Blacks were considered inferior and worthy of slavery for centuries based on Genesis 9:20-27 where Canaan  and all his descendants were cursed to be slaves because of the mistake of Canaan's father, Ham, at seeing his father's nakedness. Of course we now know that this wasn't directed at blacks, (Canaan's skin color isn't even mentioned!) but as a justification for the Israelite invasion of the land of the Canaanites.

In the same way, Biblical texts about male on male sex in the Bible are misinterpreted as modern Biblical studies and science prove yet many refuse to see that.