We are determined by biology even more than by culture, and culture is often just an attempt to work around biology in order to service our social organization's requirements for permanence and stability..
Men are programmed by evolution to spread as much of their DNA as possible in as many women as possible, and they have the biological equipment to do this. It is in their genetic interest that other men raise their offspring for them so they can maximize the propagation of their genetic material.
Women cannot reproduce every day of an entire lifetime like men, but are limited to at most a a score of offspring spread out over as many years. It takes them many years to raise a child to adulthood. A child's father could be anyone, but the mother is never in doubt, so it is in the best interest of female DNA to have access to and protection from one stable and capable male for as long as possible, whether he is the father of her children or not.
It may be unfair, but one of the prices we pay for intelligence, a long gestation and childhood, is paid primarily by the female of the species, and the instinctual reproductive strategies of the two sexes not only differ, they conflict. Our culture attempts to level the playing field somewhat with institution and ritual, (like romantic love, marriage, fidelity, etc) but it has only been able to do this by creating a state of chattel for women. They may have their security, but they've paid for it with their freedom.
Of course, technology makes much of this obsolete, but cultural structures die slowly, and biological ones slower still.