John Stuart Mill was a political philosopher who is best known for his work concerning liberalism and the ethical theory, utilitarianism. In one of his most famous books, he conceived what is known as the harm principle. This affirms that one should be allowed to do whatever they so wish, as long as it does not harm others or infringe on their rights. This can be applied to the topic of sexual ethics. Mill, using his harm principle, would assert that all forms of sexual behaviour should be tolerated providing no one is harmed.
However this does not settle the controversy surrounding sexual ethics and what is considering moral during sex and when it should take place. Traditional Christians, particularly Catholics following Aquinas natural law theory, argued that the main purpose of sex was reproduction (one of the primary precepts in Natural Law) which therefore deemed homosexual acts as immoral because they could not lead to reproduction. They also deemed pre-marital and extramarital sex as wrong and immoral because they believe that only through marriage could sex be allowed as marriage is considered by them to be the most stable way to bring up children.
Additionally extramarital sex is clearly forbidden in the Bible, thou shall not commit adultery and it breaks the commitments made during marriage vows which Christians would see as breaking the union between them and God. These beliefs however are arguably outdated and do not fit with a more secular and liberal society in which much more is acceptable than it was in the times when the Bible was written.
Traditional Christians ostensibly follow biblical rules that do not affect them and rather follow those which seeks to oppress the minority. If they believe the bible to be right on homosexuality then why do they not follow the rule that forbids them wearing mixed fibres. This proves the traditional Christian and the teachings of natural law to be inadequate and so all forms of sexual behaviour should be tolerated providing no one is harmed.
Aquinas, the main proponent of natural law believed that sex should only take place within marriage and therefore all forms of sexual behaviour should not be tolerated. The reason that this is the case is due to the belief that the true telos of sexual activity is to fulfil procreation, one of the primary precepts. Sexual acts are only good and moral if the fulfil this end. They therefore believe that homosexual acts are immoral because they cannot lead to reproduction and therefore are seen as unnatural. This does not mean that they believe homosexuality is wrong. The Catholic Church, who develop a lot of their attitudes based on natural law, believe that homosexuality is not wrong because it cannot be helped but rather homosexual acts are immoral because they can be helped. In addition the Bible says nothing about homosexuality in terms of it being a sexual orientation but rather homosexual acts “you shall not lie with a male as with a woman; it is an abomination. Pre-marital sex is forbidden under natural law because they believe that only through marriage is the best way to bring up children and because the main purpose of sex is reproduction, pre-marital sex is wrong. This adheres to the primary precepts of reproduction and education of the young. Natural law followers also forbid contraception, a secondary precept, because this prevents reproduction and the main purpose of sex should be reproduction. Extramarital sex is also forbidden by almost all Christians because the Bible explicitly states thou shall not commit adultery, with regards to natural law, this goes against divine law and so natural law followers would rule out extramarital sex without reference to the precepts. Natural law however lack sufficient reasoning to say certain sexual behaviour is wrong. Natural law is outdated and may have been valid during Aquinas time but is not reasonable to attempt to apply it in a more liberal and secular world. Natural law also fails when claiming that homosexual acts are unnatural and therefore immoral and wrong, homosexuality is present in many species not just humans and the claim that homosexual acts are not natural and therefore wrong is illogical. Many natural things are undoubtedly wrong such as cancer and many non-natural things like the use of prosthetic limbs are good, just because something is not natural does not make it wrong.