I believe that natural law is of no use when discussing sexual ethics as it is very vague and doesn’t take into account specific situations. Natural Law says that everything has a purpose, and that mankind was made by God with a specific design or objective in mind. It says that this purpose can be known through reason. As a result, fulfilling the purpose of our design is the only ‘good’ for humans. The Primary Precepts of Natural Law require us to ‘live in an ordered society’ to ‘reproduce’, to ‘educate our young’, to ‘protect the innocent’ and the most important to ‘worship God’.
The most relevant of these in this subject is reproduction, and if you look at it as a duty to continue the re-population of God’s people than of course sex can be understood as a good and moral action. In fact there are many biblic references to sex and Gods opinion on it, in Genesis He says; ‘be fruitful and increase in number’ and through-out it shows that sex is necessary.
The problem for a Natural Law supporter comes when sex is only done as an ‘efficient’ cause, e. g. or pleasure and enjoyment, this is not following God’s final cause of re-production.
Contraception is seen as immoral in Natural law and for most Catholics. If contraception is used it splits the ‘efficient’ practice of sex from the ‘final cause’ of reproduction; this goes against that actions purpose and makes it wrong as not what God intended. However some Natural Law supporters would argue that sex is not about what humans gain from it but what God actually intended it to be about, which could be unity and love etc.
Other philosophers/philosophies who argue that this way of thinking about contraception and sex can lead to situations of unnecessary suffering. For example, a utilitarian approach would assess how pain and pleasure can be maximised in a situation and so would probably sharply contrast with Natural Law. Another thing to consider when thinking about natural law and contraception is that they don’t take into account third world countries, for example, and how not using contraception will lead to a spreading of aids and children the parents can’t afford to give a good life.
Homosexuality is another sexual issue that Natural Law calls immoral for the same reason as contraception, because it doesn’t fill in sex’s final cause and reproduction and so homosexual sex cannot produce any children it is not ethical. The Catholic Church has long supported this view and have declared that homosexual orientation is not in itself immoral but just like the infertile couple sex without the possibility of children makes it wrong.
However another thing to consider as part of the ‘living in an ordered society’ precept is not being judgmental of other people. Pre-marital sex could be seen as either good or bad by natural law followers as if the couple is reproducing and being good nurturing parents then they’re fulfilling most of the primary precepts and that is good. However they aren’t fulfilling the main one, worshiping God, as the bible teaches that sex should be saved for marriage, so a lot of natural law followers would say it is wrong.
Something to consider in this is if they’re actually planning to get married at all? Does it still count as pre-marital? Extra marital sex is also a contentious issue for Natural Law followers. In a wedding ceremony promises are made before God ‘to love and to cherish’ and most importantly ‘forsaking all others’ to stay married until ‘death do us part’. As a vow before God, this cannot be broken and still be part of the primary precept ‘worship’ God.
The ultimate aim of life is to be ‘united with God’ through our moral actions, as society where immorality and deception are accepted is not a place where God is being worshipped effectively. Sex should be about reproduction and families and therefore bringing you closer to God not a violation of a sacred vow. To conclude, I would argue that Natural law is an overly harsh method of making ethical decisions. It does not take into account the most loving action for people and nor does he make any exception for circumstance or what will benefit the majority of people.