What should I do about this?

There's this guy sit next to in math, I thought he was gay. So when my friends said he was really flirting with me in math I was shocked. They say I'm always smiling and being nice around him, (I'm pretty unfriendly), and that I'm leading him on. One I don't like him, and two, I don't want a bi person to like me!!! (He DID/DOES like guys though so he's definitely not straight)