It's wrong because it's a faulty abstraction.

It's generalising. That is, assuming that one or many things are one way just because some things are one way.

Just because one person is one way, doesn't mean they all are, or even mostly are.

I hope you now see why it is FUCKING STUPID TO BE A SEXIST, racist or any other kind of generalist.

Best regards, hugs, kiss, love,

ps. sorry for the non-capital PC parts.