[Jack_]

Personally I'd say it's the way we're brought up. If you think about it, from the minute we're born we're instantly bisexual. It's not until we start to grow up that everything around us is based around opposite sex couples, our parents etc. Stories, fairytales, TV shows all base their shows around love between man and woman. We begin to believe that anything else isn't 'normal', and so thus become straight until we get older and begin to realise that it isn't just like that.

It's not until, or unless you actually think about why it exists that you start to realise that things don't just have to be about a man and a woman. For people who haven't/don't do this I think it's just a mix of what I said before and following the crowd, i.e everyone else hates them, so I will too.