Yeah, dude, that is really sexist. Sometimes a female cosplayer can make herself look more like the male version, or doesn't want to make the genderbend's more complicated costume, or just doesn't like the genderbend. Like me, I chopped off a foot of hair to cosplay the guy version of a character.
There are also girls who don't want to "be pretty in everyday life". Just like not all guys wear tuxedos with a carnation in the buttonhole every day. Yeah, it'd be nice if everyone was elegant all the time, but there are things called not getting enough sleep and rushing out the door in ratty jeans and no make-up. Not every girl is attempting desperately to attract a mate with her appearance. Sorry to break it to you, but women are not obligated to make themselves drop-dead gorgeous for your viewing pleasure while you get to go about with your pants halfway off your butt all the time.
Besides, this is cosplay. As in it's from anime. As in the men are just as damn pretty as the women. Crossplay is not doing something horrible to yourself, any more than wearing a wig is.
(An anecdote- out of curiosity I asked my straight and manly older brother: "if a very hot chick who was crossplaying a really manly guy asked you to yaoi with her, would you?" He grimaced briefly before replying "you know, that's just kinky enough.")
Past Cosplay: Death of the Endless (Sandman), Trinity (The Matrix), Austria (Axis Powers Hetalia), Orihara Izaya (Durarara!!), Mey-Rin (Black Butler)
Current Cosplay: Dodo Chaplet (Doctor Who)
Future Cosplay: Dalek dress, Mags (both also Doctor Who)
For cosplay photos please visit my tumblr