The derivation of the precepts of the Natural Law

According to the New Natural Law Theory -- Robert George gives a brief account in his response to Michael Perry:

Well, I don't want to misunderstand Michael, so let me ask: What about the absolute norm that forbids having sexual intercourse with a woman against her will or without her consent? It seems to me that there are only a couple of ways Michael could go here. One is to affirm the exceptionless norm against rape and say that it is merely a specification of the love commandment of John 13:34. If so, fine with me. I would then want to argue that there are many such specifications, including the exceptionless norm against the direct killing of innocent human beings at any stage or in any condition, and those against adultery, fornication, sodomy, and other intrinsically non-marital sexual acts. Michael might disagree with me about some or all of these, but our debate would not be about whether there is only one exceptionless moral norm. In each case, it would be a debate about whether a particular norm is exceptionless, or a valid norm at all.