Followng on from Dave's post - I don't see anything odd about males writing stories with female protagonists, or females writing stories with male protagonists. People who think otherwise might have an ideological take on the issue, but I mostly blame the notion that fiction-writing is a form of autobiography. As a fantasy writer, I aim to create interesting characters, not convey comprehensive pictures of detailed everyday life. It's true, there are some aspects of life as a woman that I couldn't handle very well - but there's so much left. Fictional characters are only a selection from anybody's full life anyway, aren't they?
If a male author can't write female protagonists and a female can't write male, the logic of the argument ends up with no author allowed to write any character outside their own personal experience - ultimately, your only protagonist is yourself. That's not how I write. I mean, I use bits of myself in different characters, but only ever bits. I don't want to create characters who are just duplicate Richard Harlands. How boring! How self-obsessed! I like to start with OTHER people, and then try to recreate them from inside.