I mean keep in mind focusing on your own children doesn't mean not working with others, hence a "village", it just means you prioritise your own children relatively speaking (hence encouraging the monogamous focus for the core relationship, followed by the community as the secondary relationship). Also to an extent yes it is a prisoners dilemma, their can always theoretically be a purely altruistic way of doing... but the issue is coming to it naturally. After all we are not talking about modern day society, we are talking about the history and origins of marriage and the way that the arrangement came to be. In an alterustic group, if any one parent focuses on their own children then they will survive more often and thus those who act such a way will do better as they benefit from the others altruism. So naturally they succeed more and populate more.