I know a girl who I used to be friends with, and she's always saying that being gay/lesbian is a disease, and a choice. I'm bi, and it makes me feel like something is wrong with me.
There is nothing wrong with you. Don’t ever let someone make you believe this. Being gay/bisexual is not a disease, and whatever she says should be irrelevant.
Don’t worry about her, and don’t let in people who makes you feel this way in your life, so please don’t worry.
Being bisexual doesn’t make you a worse or a better person than anyone else, you’ve chosen a different way to love/like people, and difference should be embraced. :)