In The Evolution of Social Monogamy in Mammals, Lukas and Clutton- Brock dismiss the previous theory that social monogamy was a result of a need for extra parental care. They say, with a lot of statistics to back them up, that the actual reason for the evolution of social monogamy is the decentralization of females in a territory. What most likely happened was female competition prevented formation of high-densities of females in territories “controlled” by males. This mean that males would essentially have to choose a female to protect, and from there social monogamy became common. On the other hand, in Male infanticide leads to social monogamy in primates argues that it was in essence the need for extra parental care to ensure the survival of offspring led to social monogamy; that a need for bi-parental care led to this phenomenon.
I personally agree with Luka/Clutton- Brock. Mammalian males ancestor used to protect land, not a female and his offspring specifically. As tension between females developed, then the male was forced to take care of the single female in his territory. These relationships are not very common in wildlife, and so it does not make sense that they were a cognizant change made by the males.