Matt Yglesias and Nick Beaudrot make good points about women in Congress (minor quibble: Beaudrot has the number wrong; about 17% of Members of the House and exactly 17% of Senators are women).
What's worth noting, however, is that this is increasingly a partisan difference. Over a fifth (22%) of Democrats in Congress are women (same percentage in both Houses). That's not 50%, but it's a large and growing number. On the GOP side, only four of the forty Senators are women, and just under one tenth of the Republican Members of the House are women. This is a relatively recent phenomenon. Throughout the 1980s, women in Congress were almost evenly split between the parties. And then the number of Republican women in Congress basically stagnated (14 total in 1989-1990, 21 total now), while the number of Democratic women has exploded (17 then, 69 now).
Not just Congress. While Governors are split three each between the parties, there are 47 other statewide elected Democratic women, and eighteen Republicans. Seventy percent of women in state legislatures are women. All ten of the women in top state legislative positions are Democrats. (All these numbers taken from the invaluable Center for American Women and Politics).
So while liberals who believe in greater equality should certainly consider supporting the Women's Campaign Forum's "She Should Run" efforts -- I think it's a great idea -- the biggest barrier to women in politics these days is really the Republican choice to revert to a white, Christian, male set of office-holders (yes, Republicans have also stopped sending Jews to Congress, and it's not as if they ever sent Muslims or athiests...).