I've taken some big cocks balls deep, but there are *so* many factors. Thickness, angle, cockhead size....
I run into problems when a dude's hard shaft wants to stick close to his belly. Some guys have dicks that just lean back into themselves like that - or pull up tight that way.
But I've been with a black dude who had a legitimate 9.5er and I took him balls deep without even blinking...partly because his cock had no curve to it and was really straight as it projected out from him.
I don't know that a woman couldn't deepthroat as well as a man.....again, multiple factors. A, what's the willingness factor? And B, some guys like very petite women. It's hard to lay ten inches of pipe into an esophagus that is smaller than standard size. It would be like Yao Ming trying to drive a Cooper Mini, y'know?