The necessity of avoiding doing something wrong in order to achieve some good is not a natural law imperative. It's scriptural. So, for example, whatever one's circumstances, contraception, abortion, euthanasia, homosexual sex, adultery and fornication, judged as sin in and of itself, regardless of intention, is always wrong. 

Natural law is a way of understanding and explaining God's Law that isn't solely reliant on Divine revelation but is assisted by insights from creation and the application of reason and logic, based on the premise that God's is rational and His creation therefore has intelligible purpose and meaning.