Then what's with the "Proud to be LGBT" thread?
After all it's just a preference.
Well, when your entire culture tells you that you are different, people find that they need to remind themselves that their identity is not something to be ashamed of, but something to be proud of, something that is a part of them to be embraced.
It's not just a preference, it's a way to combat oppression. It combats the idea from the 20th century that being homosexual was a deviant illness (it's where the term homosexual comes from) to be treated by chemical castration or brainwashing. The "Scared Straight" style cults used to be much more than fringe religious groups, but intertwined with our science and medical conclusions.
Being straight is just a preference, because we don't have to come out to anyone. It's assumed you're straight - unless you talk, walk, or dress differently enough that people will make different assumptions - you don't have to do anything to have your sexual identity accepted by the culture around you.
I am not proud to be straight, because so many people like me use their sexuality to oppress other people, and try to make them feel bad about being public with who they are, ignoring that nearly every movie and TV show features many heterosexual relationships without the need to make it the topic of the show. They judge people for acting, looking, or talking differently. They make assumptions that the sex you like the most defines your entire person.
I am straight and incredibly appreciative that I happen to fit into the majority and don't have to struggle with that part of my identity except when I choose to.