There it is what I hinted to in some previous posts: natural law isn't but charitas.
The priest was explaining the meaning of charitas with words that didn't have any specifically christian about them. Because there isn't anything specifically christian in love.
English is rather poor on this regard. It doesn't know other word than love, other declaration but I love you.
In Italian, besides ti amo, of use among lovers in the tecnhical sense of the word, we have also the more generic ti voglio bene: literally, "I want good to you".
It could work as a definition of love. But it also says everything there is to know about natural law.
To declare our love to someone is a rather paradoxical affair. When we do it, we make known how much we care for him or her, even how much we are in need of that person, that as a matter of fact we cannot do without her. But woe to us if we say it. We have to say the opposite: I don't think of my good, but of yours. Why so?
There is a simple fact about human nature to keep in mind: we cannot give ourselves our own good . I don't exemplify. Just think of it.
That's all there is to your great discovery? you might ask. It looks more like discovering hot water.
Mind me, I said "give", not "take".
What sets man apart from the remaing animal world, is that, being endowed with reason, he knows his needs are addressed to others like him, equally in need. Neither can take, but they con only wait to receive, provided that they are reciprocally well disposed. Nobody can force giving, without changing it into taking: meaning, by violence.
Of course we can always buy what we need. Provided it can be sold. And even then, we must be able to rely on the seller. There must be trust among us. Call it bonam fidem: necessary condition according to Roman jurists for the validity of contracts. Unless we think an adequate substitute fear of punishment, i.e. violence, coming from the one sovreign to whom we have granted the right to keep us in line: a police State.
This is then man's nature, and therefore the law governing relations among men and women: either they live together in liberty as friends, or are potential enemies kept together by fear.
Christian fides is there to remind us of what is just: the friendship natural to us.