Presumably any man who is named as the father of a child has slept with the mother. So the fact remains that he has been willing to become a father (or run the risk of becoming one) in the first place. If he has then gone on to play the role of father to the child, he has then had the benefit of both the sex and the affection of the child. What possible excuse can he have for not paying? At the end of the day, the man has had his sex, his child and the slings and arrows of outrageous biological (mis)/fortunes are no excuse for him to damage the family, which has chosen him.