It isn't just road cycling is it - it's all forms of cycling. For that reason I don't really think it's down to cycling clubs - it's just that cycling doesn't tend to appeal so much to so many women.
Yes there is a bit of a viscious circle in that there being fewer women in clubs means they cater less for women - but the root problem seems to be that more men like riding bikes than women.
Maybe it's the risk factor - real or perceived - but then there are other high risk (relatively - no sport is really high risk) sports like climbing and I get the impression that is not unpopular with women.
Triathlon seems to be bringing more women into riding road bikes - but I think they tend to come into it from running or maybe swimming and they aren't getting into road racing (on bikes) - there are very very few women doing that.