I've heard it all: women aren't as aggressive (there is an inherent connection between theoretical reasoning and aggression?), women prefer jobs that aren't this hard, women just aren't as good at abstract thought...
All are theories that should not be ignored, but not prioritized either...not when there is such good evidence (from our discipline and others) for the role of social factors.
Jenny Saul blogs it well here.
In addition to the social factors covered in that post, the Feminist Philosophers blog has over the years cataloged many a conference and edited book that inexplicably includes only male authors. JJ suggests that networking may be not just the problem but also the solution.