The life expectancy back then was not very high. Few got older than 30. Moreover the offspring incubation cycle of female humans compared to other mammals is extremely long. In essence this means that the amount of offspring a female could manufacture over its lifetime was extremely limited. Wasting production capacity on inferior males would produce inferior offspring with sub-standard survability, compared to those who select their subcontractor (human males) carefully. In short, the offspring of non-promiscuous females had better survivability than those of the promiscuous females, and the non-promicious female offspring was in turn more likely to produce offspring itself without dying before fulfilling its purpose.