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After discovery and description of Ardipithecus ramidus there is opinion that humans evolved as monogamous species gradually because it was more advantageous. Here also goes explanation of cryptic (hidden) ovulation and sex throughout.
It is very interesting and deep topic and cannot be explained in one short answer.
Cut and paste from here:
"In apes and monkeys, the male's upper canine tooth usually has a prominent, dagger-like crown that is continuously sharpened (honed) by wear against a specialized lower premolar tooth. Together, these teeth form so-called C/P3 complex, also referred as sectorial canine complex (SCC). This complex was developed in the course of evolution not as a dietary adaptation but rather as as a formidable weapon in various social conflicts. Size of the canines also reflects status of the individual in the social hierarchy. Males demonstrate their canines by gaping widely at each other thus establishing their positions in the group. All fossil apes had the honing C/P3 complex.
AGGRESSIVE MALES WATCH OVER HAREM
In contrast, Ar. ramidus has dramatically "feminized" canines: they are shorter, diamond-shaped and there is no evidence of honing. The hominid canines from about 6 million years ago are similar in size to those of Ar. ramidus. Australopithecus afarensis had more robust canines but honing was absent. This suggests that male canine size and prominence were reduced by ~6 to 4.4 million years ago from an ancestral ape with a honing C/P3 complex.
Subdued shape of canines in Ar. ramidus implies that selection, probably, sexual, played an important role in elimination of agonistic behavior observed in monkeys and extant apes.
*****Thus, fundamental reproductive and social behavioral
changes probably took place in groups of hominids long before their brain started to enlarge. ********
Reduced aggressiveness in males facilitated bond formation between males and females. Males had to help chosen females in bringing up their offspring and change in sociaal organization was needed - hence monogamy.
In nature magazine authors maintain that "sex for food" at some point became a routine.
How come that females started to choose "weaker" less aggressive males?
"Possible mechanism of sexual selection might be gleaned from sexual behavior and secondary sexual characteristics of male gelada baboons. A handful of males grow rapidly at puberty, develop distinctive golden manes and become almost twice the size of the females. These males usually succeed in acquiring their own harems. After a relatively brief but intense reproductive life and constant conflicts with rivals, they are displaced by younger stronger males and never breed again. However, when many such males are present in the social group, an adolescent male may adopt a distinctly different phenotype and behavior - he remains female-like in size and appearance, does not develop the mane and acts as inconspicuous as possible. Such males hang around the big males' harems, mating with females when harem-owner is distracted. Because small and sneaky male never has to fight for females, he is likely to have longer, if less intense sexual life, and in the long run he even may achieve better reproductive success than the aggressive male. "
 Posted 234 days ago
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