>There's nothing about gender in an of itself that makes you drive badly, even if 80% of women get together at a secret meeting and agree to drive badly. In that case, the 20% would be suffering discrimination purely because of their gender.
Look at it the other way around. By not taking it into account, 100% of men would be suffering because of the 80% of women.
In any case, this isn't any different with all other factors. Age and profession for example. There are really good 16 year old drivers, why should they have to pay higher insurance? By your logic making any predictions is wrong if they aren't 100% accurate. Gender is just a subcase.