My general impression (and I stress general, so please, no hate mail) here in Colorado is that women are more inclined to use cycling as a compliment to their overall fitness/recreation, whereas men (due to the fact that cycling has less of a cool factor than climbing etc...) if they do cycle, are really into it, either eventually becoming competitive or at least internally competing against their peers on group rides.
And this I feel supports the above posters' assertions that cycling is not good for losing weight. Face it, it is not the most efficient way of losing weight (running will burn more calories in a shorter period of time). In addition, it is possible to ride a bike, feel like you are getting a workout, but in reality, you are doing a whole lot. I see it on the bikepath. People who spin a little, then stop and coast. Sure - you are moving, but are you really getting the heart rate up?
So someone like me, who will ride upwards of 170 miles in a typical weekend, will have trouble keeping my weight. And trust me, there are women who ride like that, and kick my but up the front range.