We are all very well aware of the main reason people are against gay marriage, The Bible. Christians talk like they have to live life exactly by the book (which they rarely do) but there are parts that even they miss.
Leviticus 20:13 - "If a man lies with a man as one lies with a woman, both of them have done what is detestable. They must be put to death; their blood will be on their own heads."
Why don't I see any Christians campaigning for the slaughter of anyone who is caught having same-sex relations? Consistency people. If you are going to follow some of it follow all of it that is what a true believer does.