Because they're not correct for everyone. Replace gay stereotypes with race stereotypes or cultural stereotypes and it's the same problem. Also It's a generalisation and affirmation bias leads people to notice every gay person who fits stereotypes, yet doesn't account for gay people who don't because sexuality is invisible (unless they literally see you fucking the same sex) so a person decides all lesbians act a certain way as they don't bring it up if they don't suspect it based on a visual clue, most of the time. It ends up being that the vocal minority which fits stereotypes represents the whole group. That's a problem because straights who just happen to like those things get branded as gay, and gay people who don't feel like they can't really be gay. Both are bigger deals for adolescents coming to terms with their identity. The only thing which should define being homosexual is same sex attraction, anything else will just cause issues.