The human mind is very interesting in this regard, I find. I've put much time into that topic myself, talking to different kind of people and reading in a lot of different forums and whatnot.
It seems that most humans have an incredible drive to control the life of other people. May it be their personal beliefs and opinions, their taste in music, game, clothing, food, their sexuality, and what not else. In fact, religious debates and talk about equal rights for homosexuals is where you can see that the best.
People who are against, let's take latter as an example cause I know it best, homosexuals being treated equally are forcing their personal beliefs and opinion onto people who are not of that beliefs/opinions. This goes so far that they in the end are controlling what you have to like, and what you don't have to like, so in result there are people of beliefs that you may not share, but they are still controlling your life.