Natural Law is a doctrine designed to give the Vatican wiggle room to advance its policies. It's used to add authority to moral rules that can be found in scriptures, and to justify ones that can't. “Natural law is allegedly the law of God made known to reason. In effect, it is a free ticket for religious authorities to claim they ... can read God's mind.”  As Church doctrine this is no one else's business, but a problem arises when they try to impose these “laws” on the general public.
"Natural Law" is not the laws of nature which are discovered by science and tested for their truth. It's a totally unprovable entity, supposedly the moral sense written by God in the hearts of all mankind. A Catholic theologian's book about it is titled What We Can't Not Know. A reviewer remarks that Its author "finds fault with every ethical system but his own because they fail to account for this natural, absolute law written in our hearts."
In practice, Natural Law tends to reflect the political goals of the Vatican and it often exerts pressure on countries to conform to its current version. Prof. Guenther Lewy explains how this works:
Natural law is abstract and vague to the point of making its application to concrete cases extremely difficult. It requires an authoritative interpreter, the Church. But this means that the gap between the abstract principles and the case at issue will be closed by answers tied up and almost predetermined by the interests of the Church as an institution. […]
♦ The German Catholics in 1937 were told that resistance to the Nazi state was sinful;
♦ Spanish Catholics at the same time were urged to support the rebellion of General Franco against the Second Spanish Republic.
♦ In World War II Catholics serving in both of the warring factions were assured that they were fighting a just war.
♦ The institution of slavery was at one time defended by leading theologians as in consonance with natural law [see Papal permission to convert by force and enslave pagans]; today, in an age of colonial revolts and assaults upon all forms of discrimination, the Church stands for full equality.
All these positions have been justified on the basis of the same natural law premises. […] As a Protestant theologian, John C. Bennett, has expressed it […], “Natural law plus prudence equals flexibility”. 
Because Natural Law is supposed to be universal, popes try to prescribe it for everyone, Catholic or not. Thus when Benedict XVI announced his forthcoming trip to the UK he took the occasion to scold the British Government for introducing equality legislation which was “against Natural Law”  Three days later the UK Government backed down and withdrew it. 
1. Robert Todd Carroll, “Natural Law, Celibate Men Who Wear Dresses, and Pastors Who Hate”, Skeptimedia, 4 February 2010. http://www.skepdic.com/skeptimedia/skeptimedia71.html
2. Guenther Lewy, "Catholic political ideology: the union of theory and practice" , Chapter 12 (the final one) of The Catholic Church and Nazi Germany, 1964. http://www.mosquitonet.com/~prewett/lewy337341.html
3. Ruth Gledhill, “Pope Benedict XVI attacks Labour's equality push”, Times, 1 February 2010. http://www.timesonline.co.uk/tol/comment/faith/article7011296.ece
4. Emily Ashton, Discimination law change is dropped after Pope's opposition, Press Association, 4 February 2010. http://www.independent.co.uk/news/uk/politics/discimination-law-change-is-dropped-after-popes-opposition-1889647.html