It's dirty work, but somebody's gotta do it.

What does this have to do with anything? My initial point was that while individual liberties may be locally beneficial for the individual, they may also be non-beneficial for society as a whole, which, in turn, affects their personal benefits. I dispute not the claim that selfishness is natural, but that it is necessarily helpful. I cite your own example and interpret it in this light. Women who, pursuing their own benefits, choose not to have children are part of the reason why women with many children are poor. You don't seem to dispute this, but argue that the logical solution is for no one to have many children or for no one to have children early. I do not see how this is a logical outcome of the premises.