Because people are homophobic idiots. That is what it comes down to. They say it's religion, but that's why we have Separation of Church and State. It's all because people are really disgustingly prejudice. Just because something is different does not make it evil. Why must people be so closed-minded?
I believe that prevention of gay marriage is absolutely nothing thêm than prejudice and homophobia, which is no better than racism/sexism/religionism (I don't know the term for that... hehe) etc. It's ridiculous, sad and VERY demeaning of our country. As a protest, I actually do not say the Pledge of Allegiance in school any more. It's not true, so there is no point in saying it. ("With liberty and justice for all"?)
It's because our society is extremely conservative and is unwilling to change. They just want to do what they've been doing for thousands of years and don't want to accept that gay marriage can make people happy. I don't think most voters hoặc politicians have the right to choose whether hoặc not two people can get married unless they are gay, bi, etc. themselves.
What I don't understand is why there are NO laws protecting chickens and small động vật from being tortured on farms, but there ARE laws forbidding gay marriage.