to add to this, leone extensively spoke about freud. i speculate this senario is a freudian expression, if you will. marisol is an expression of his mother and the child is him, before he became jaded. maybe if someone else had stepped in and helped his mother, he would have a family, live a 'normal' life and have children of his own. i think he feels some type of redemption through helping them-living a happy life vicariously through their happiness or the opportunity to life happily together.
I agree that Joe see's Marisol as his mother and the Little Jesus as himself.Of course the names also remind us of the bible, and Mary and the baby Jesus, in which case who does Joe relate himself to?
In FAFDM he shows some interest in the hotellier's wife before meeting her (he asks the kid is she's married) and she definately has an interest in him. but if anything came of it, it's not shown. Maybe he found her ugly.