we are promiscuous.

our male ancestors that mated with more than one female had more offspring than the males that mated with only one female. this of course produced more children that were the result of promiscuity than children that were the result of monogamy. the children of the promiscuous father most likely inherited his promiscuity and produced even more promiscuous offspring. thus more and more humans would either mate with or be the result of a promiscuous human and would pass on that trait.

females are promiscuous because it allowed them to shack-up with a caring male that was a good provider while they had the baby of a more virile male that created stronger, better surviving, and more promiscuous offspring. a virile and promiscuous male would want to spread his seed around so he would not stay long to take care of his children or their mothers.

and yes females would have mated with caring males that were good providers. but the ones who did, produced children that were monogamous. the ones that cheated on their good provider and mated with promiscuous males had the better advantage. they had a good provider and promiscuous genes so their offspring were provided for and went off and produced more promiscuous offspring.