Well, I mean, if you'd rather a political than a mechanics-of-law explanation as to why the law treats two situations differently mo, it's legal because heterosexual couples plus women outside of such couples combine to form a powerful voting bloc, and gender rating, compared to it's absence with no other change in practice, is a benefit to that group.

Specifically, compared to perfect allocation of costs, it's a small subsidy from people outside of opposite-sex couples to opposite-sex couples, whereas not having gender rating with otherwise status quo policies is a large subsidy from women not in opposite-sex couples to men not in opposite-sex couples.

Under status quo policies, opposite sex couples are in practice fairly free to gender-optimize insurance pricing by putting the less expensive to insure driver as primary on the more expensive to insure vehicle, regardless of actual primary driver.

I feel like this is just a good way to make women pay as much as men, instead of lowering prices for men to pay as much as women.

But no one said it is was ethical in the case where it is legal, so if that was the intended question, not only is it phrased wrong but it starts from an unsupported premise, to wit, that people in the discussion believe that one is ethical and the other not.