Girls and gaming 0_o?

Why do guys assume there are no girl gamers that are actually good? Why do they also assume all girls play the sims games? Personally on the sims I get bored after 5 minutes and always end up burning everyone around me (: and I prefer games like halo, assassins creed, oblivion ( extremely happy I finally found it for ps3) so could someone please explain the massive stereotype to me?