In the United States, about 23% of all cyclists are women (which is based on Census data about commutes). In fact, the number of women who commute by bicycle has decreased from from 33% of all bike trips in 2001 to 24% in 2009 (this is from the American Community Survey, which is conducted by the Census Bureau). For comparison, 55% of all bicycle trips in the Netherlands is by women, and 49% in Germany.
The above data about the US is especially striking if you consider that bike trips are up significantly during the same period of time (2001 to 2009), which indicates that many more men are taking to the streets by bike but women are not.
There's something different about the United States. And it's infrastructure. Studies across all sorts of disciplines show that women are more risk-averse, and indeed, concerns about riding in traffic are overwhelmingly the reason women cite for not riding their bikes. Women also have different travel patterns: they perform the vast majority (77%) of all "serve passenger" trips (hauling people, usually children, around), engage in more "trip chaining," and run more errands, like grocery shopping. They also drive fewer miles to their commutes (this is all based on the 2009 National Household Transportation Survey).
And where have we invested in segregated bicycle lanes? Along recreational corridors, that don't lead to schools, daycare centers, grocery stores, or major employment centers. In other words, the safest bicycle facilities have been installed where few women really want or need to go.
If we're going to increase bicycling—which everyone agrees at least theoretically is in the community's best interests—then we need to invest in safer infrastructure to serve the population that isn't young, male, and super fit. If women are bicycling in equivalent numbers to men, then the revolution will be here.
I'm a graduate student in urban planning at Eastern Washington University, and I'm writing my thesis on this very topic. I have a shitfuckton of data and academic papers at my fingertips, if you want more info.
Here's a picture of my decidely unfit self, my incredibly cute and awesome daughter, and the kid-hauling machine we use to get all around Spokane. I even have studded tires. I'd sure like some better infrastructure, and every time I tuck my daughter into that trailer I wonder if I'm making the right decision. How will I live with myself if something happens to us?