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. for 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