Yes, society likes to put everything into little packages because it makes the world easier to understand. But not everyone fits into a pre-set package (in fact, a lot of people don't).
People see you as a lesbian now because you are in a relationship with a woman. If tomorrow you broke up and you started dating a man, the world would see you as straight.
It's like if you saw a guy standing on the street corner dressed as a police officer. That's what the world will see him as. He may also be a father, or a juggler, or a volunteer at a soup kitchen, or a painter, or a war veteran. People who don't know him don't see that. They just see the uniform.
My point is, don't worry about labels of people who don't know you.
Why do you mind being labeled a lesbian? Would you like it any better to be labeled straight or bisexual?
As for the whole porn thing, don't worry about it. I wouldn't say it's "usual" or "common," but if something's not "normal" makes it sound like is bad. Just because you like something unexpected doesn't make it a bad thing. It's only a bad thing if it bothers you or your girlfriend, then it might be something to work out and understand. I think your best friend needs to open his mind a bit.