A similar norm I often see from caregivers is that they consider boys to be "rough and tumble" and girls "soft and dainty". It is easy to see how those behaviour classes could extend to insurance risk.
Is being a woman/man really the causation? Or is it something like the environment that the genders are brought up in? If it is the latter, how is that any different than a minority being brought up in a poor neighbourhood (and, statistically, stay there)? After all, in both cases, you are penalizing those who had an upbringing that is outside of the statistical norm due to a presence of correlation.