Men spread seed, there's no need for monogamy when you can ensure your genetics pass on so easily. For women, it's quality over quantity. The best chance of the survival of her offspring necessitates the best genetics she can get, during ovulation. The only time at which monogamy is natural for women is if they are with the best genetic match they can attract, otherwise, monogamy is not a sound biological imperative strategy. Now, most people might jump to purely physicality in that case but humans have evolved intelligence as much as physicality. Anyway, hypergamy selects the highest possible mate during ovulation. Monogamy doesn't support that strategy, ergo, from a biological imperative, monogamy is not natural for females.