Studies have shown that people have a tendency to -unconsciously- evaluate pieces of writing differently depending on whether they think the author is male or female. That's something beyond the overt sexism we often see in the field, and it's something that even the very best of intentions doesn't mitigate.
This is why one needs to either strip names off manuscripts or else be aware of this phenomenon in order to make sure you're not succumbing to it. It's not difficult to perform a spot check every once in a while and look for gender-related patterns and it's awesome to see more and more people being aware that they exist. And isn't it something one wants to know if they're doing it, much as one would prefer that a friend discreetly nudge you and mention that you've got a bit of spinach between your teeth rather than let you keep talking away?
It's also discouraging to see intelligent people unwilling to look at their own biases. It's not a case of snapping in the "I am now feminist" filter and seeing and being affected by the world in a different way. It's a need for willingness to examine your perceptions and admit how much one is shaped by our determinedly hegemonic culture. We do not exist outside that culture -- we swim in it all the time. Look at crazy Rachel Moss and the SASS crap -- that's the dominant culture doing its best to impose itself on something that directly challenges some of its core assumptions.
This ties into an ex's theory that I've always liked about the necessity of acknowledging and examining your own biases. We called it "Acknowledging Your Inner Shittiness" and it's coming to the realization that you're human, that you have biases, and that like every other human being on the face of the earth, you've done crappy things on occasion for ignoble or petty reasons. Because unless you're willing to acknowledge that, you're never going to be able to try to move beyond it. Or be able to look at your own actions and see where you need to amend them. And for writers, how can you understand your characters' flaws if you have none of your own?
I think I've told the story before of the job interview where I asked the interviewee to talk about a time she failed and what she did about it and she replied she couldn't because she'd never failed. If there was a single question that sank her, that would be it. Because either she was the most self-deluded person I'd met in a while, or she was some sort of freakish alien, and in either case I didn't want to work with her. I'm pretty sure she thinks she has no gender biases too.
In past iterations of this discussion, people have offered a theory of "boy" stories and "girl" stories, with the premise that "boy" stories are more likely to appeal to male editors, who then favor them over "girl" stories. I don't think it's that simple, but I'm going to leave that for another time.
The links I inserted aren't showing up for me, so here they are in case they're not showing up for other people.
The current debate: http://joesherry.blogspot.com/2008/06/one-woman-really.html
Rachel Moss: http://theangryblackwoman.wordpress.com/2008/05/28/what-rachel-moss-did/