Not horrible people, I just don't agree with them.
This exactly.
People's social views are generally defined by religion and social surroundings. So I respect that and can understand people's views on the issue to a degree. With that said, I respectfully disagree with them. I won't vote.
I'm not talking about people who oppose same-sex marriage though. I'm talking about ones who claim to not be anti-gay and are even "OMG BUT I SUPPORT CIVIL UNIONS!!" but are still anti-gay marriage for absurdly worded Moderate Hero reasons, often involving those exact words "redefining marriage". This is really illogical and even more irksome than people who are just out and out anti-gay especially since the reasoning is often "I'm not anti-gay but I don't want to offend people who are." See bedstuy's post.
And please don't give me the crap that Obama actually sincerely opposed gay marriage prior to 2012 and had an actual "revelation" in 2012 rather than his official position just being based only on political concerns. In this case too I'm talking about people who aren't politicians or public figures and thus have no excuse.
The reason people oppose redefining marriage is because the definition in the bible is the union between a man and a woman. This is an example of classic conservatism, or trying to keep things from changing. The reason these same people support civil unions is because it doesn't change the definition. For people you describe as not being anti-gay but only support civil unions, maybe their religion or family politics has something to do with that. What they're probably saying is I don't want to be anti-gay but I also don't support pro-gay policies. So in short, people who are obviously on the right, but don't want to be called homophobes by the left.