not necissarily.
i dont have the exact demographics, but i'd say that my team (which has about 70 people or so in it) is comprised of about half girls. and NO, not all of them are related in public relations.
half of our pit crew is female (usually its completely girl-dominated) and a lot of the people working on machining and wiring and all of that wonderful stuff is about even in a boys to girls ratio.
i guess the reason why Robotics seems more male-dominated is because society generally expects guys to go out and build robots, and do manly things like that... yet for women they expect for us to grow up to design clothes or something like that. not that there is anything wrong with that, but its more acceptable if a girl becomes an interior designer rather than a guy, and a guy becomes a mechanical engineer instead of a girl. i guess thats unfortunately how life works... but thankfully, FIRST is working to change that sterotype.
personally, i love all aspects of FIRST. i'd love to be on the Pit Crew and work on the robot--hardly anything facinates me more than how things work (future Physics major here)--yet because i've got strong stills in communication and leadership and writing, i'm stuck doing a lot of the Public Relations and Chairmans Award things. its not that i Mind terribly, because i don't.... i guess it just has to do with individual strengths and weaknesses of each individual and like what Jimbo5051 said, their carreer choices. but FIRST definitely can help change those... i know if i had not joined FIRST i probably would have gone on in life to be a boring Author... but working side-by-side with NASA engineers, i've reevaluated what i want to do with my life.
but i'm ranting... this is just my two cents, hope you enjoyed it.