I'm a woman, so I must think irrationally.
I get good grades, so I must be a nerd.
I'm religious, so I must shove my beliefs down your throat.
I'm republican, so I must not care about poor people.
I'm from the South, so I must be white trash.
I'm a teenager, so I must drink and do drugs.
I'm young, so I must be naive.
I'm white, so I must be a nagging, steal your money kind of girlfriend.
I look happy, so I must be hiding everything bad
I'm Mormon, so I can't have fun.

Most taken from [link] who has a lovely complete list of stereotypes from which to choose.