It's not economically feasible for a woman to continue having children while the male can leave and mate with another woman. A woman can only have 1 child at a time while a male can get many women pregnant. More responsibility falls onto the woman and that is why traditionally (and even now in the modern days) women choose a partner and sticks with them. If a white woman picks a non-white partner, then they will have mixed children going forward. Her white lineage is destroyed.