Strategic pluralism or dual mating strategy is a theory in evolutionary biology that substantiates on female hypergamy, suggesting that women evolved to evaluate men in two categories: whether they are reliable long term providers, and whether they contain high quality genes.
One particular UCLA study for example states that, “a great deal of the evidence indicates two overlapping suites of psychological adaptations in women: those for securing long-term, cooperative social partnerships for rearing children and those for pursuing a dual-mating strategy in which women secure a social partner and engage in selective sexual affairs to gain access to good genes for offspring”. The lack of loyalty with a dual-mating strategy begets the feminine imperative.
More recently, the dual-mating strategy fell in disfavor in the scientific community for a number of reasons. One reason is that non-paternity rates are globally very low, even though contraceptives and relaxed marital norms should make AF/BB occur much more often if it was natural. E.g. only 3% of all children in the U.S. live with a step father.