I am in an unenviable position right from the off as if you accept that Christianity is the one true religion then what Christianity defines as wrong is, by definition, immoral. I also have the problem that I view morality as subjective and personal therefore I have the additional responsibility to show that there might be a sexual morality that most reasonable people would find unobjectionable without the Christian worldview. To that end I require any readers to leave their unshakable, unquestioning Christian belief at the door and read what follows as an open-minded human being.
My atheistic worldview and rational thought are in agreement with the religious criticisms of atheism: I define morality as a personal, subjective beast. A position I make clear in the post here. I think there are common, human morality that most people would agree with. It is important that people agree as morality often defines laws and laws have to be popular or they don't get passed. I also think there are intellectually inconsistent morals within Christian doctrine and there are also things that are biologically self-evident that Christianity says are wrong and I will use these to show how Christian sexual morality is not only misguided but harmful.
Christianity takes a lot of its laws from the Old Testament some of which relate to sexual morality. The monogamous nature of Christian doctrine is completely at odds with most of the Old Testament where polygamy was common, it is also at odds with the vast majority of human civilisations that have ever existed - not that that makes it right, simply an observation.
Perhaps the most egregious example of Old Testament Christian sexual immorality is the rule of not masturbating. Taken from an unrelated tale of a man, Onan from who we derive the term onanism, who would not impregnate his dead brother's widow (more on that type of thing later!) and spilled his seed on the ground. Ironically this is the Catholic Church's preferred birth control method rather than a barrier. Advanced primates use masturbation in nature, it is a personal act that impacts no-one else. How can this possibly be considered immoral? Surely anyone without the disadvantages of a religious worldview can see that it is a perfectly healthy, perfectly natural voyage of self-discovery that enables people to become more competent and confident lovers.
Moving on from 'crimes' that harm no-one on to the Christian idea that sex can only take place within marriage. Let's start with a really simple truth - sex is not a bad thing. Moving on, sex between consenting adults is not a bad thing. More importantly, sex between consenting adults is not my business, not your business and, unless you are god, not for you to judge. Before humans had the concept of marriage was all sex wrong? If so why did people, with their ability to see an 'objective' moral wrong, not refuse to do it? Is it not more likely that it is a subjective wrong that certain worldviews brought to the community? If I refuse to get married for whatever reason, and I have a few, does that mean you think it wrong for me ever to have sex? My personal view of morally wrong involves another party experiencing harm or loss. Explain to me where the 'wrong' is in two consenting adults engaging in a mutually pleasurable activity that impacts no-one else?
Let's move on to the most recently socially accepted, but not by many Christians, crime - homosexuality. Homosexuality occurs in many species in nature, again that doesn't make it right, but it stops the old argument that it goes against nature. Much like the argument above, who does it harm? If there is no harm there is no moral wrong. Christianity's outlawing of what two people get up to in private has led to many deaths, and many other crimes, for what appears to be a small offence in the Old Testament, similar to eating shellfish or wearing clothes made from two different cloths. Quite a lot was an abomination unto the Lord in that book. I am sorry to have such a short piece on homosexuality, but it all boils down to whatever two consenting adults choose to get up to in the bedroom is no business of mine. That I feel uncomfortable when seeing two men kissing (but not two girls) does not mean I have the right to call it wrong or to make it illegal. I feel exactly the same about two ugly people kissing.
What follows is pretty much off the top of my head, and you are more than welcome to call it allegorical, but the morality tale within still requires explanation. Adam and Eve were the only two humans alive. They were as closely related as it is possible for two humans of different genders to be; she was made from Adam's rib. They had children and that could be considered incest. Their two sons had offspring, but as there was only one woman around that definitely was incest. When Lot escaped from Sodom with his two daughters (after offering them up to a horny crowd to save two angels) they thought they were the last humans alive so got him drunk and tried to get pregnant by him. Noah and his family went on the ark after the flood. Again the only way to have kids in that scenario is incest, admittedly first cousins, but still... And again back to Onan and similar tales, they paint a picture of a very male dominated society where women are good for little but breeding. The rules also state that it is wrong to have sex (or any contact!) with a woman for a week after her period. Can anyone in the 21st century western world still believe that?
Finally, my biggest problem with Christianity, thought crime. Lust, for anyone other than your wife, is considered a sin in Christianity.
But I say to you that whoever looks at a woman to lust for her has already committed adultery with her in his heart.Sorry, but this is an evil doctrine. What the base parts of my brain want are not under my control. My actions are mostly under conscious control so I will take responsibility for those, but thoughts are a different level. To say thinking about a beautiful woman (or man) is a sin is to deny the human psyche. Where is the morality in punishing people for some random thought floating through their heads? I have had thoughts of killing, raping and torturing people - I was writing a book - should I be punished for those? What goes on in my head not what I should be judged on, judge me on deeds, judge me on the ideas I espouse, but do not judge me for my thoughts. My thoughts are mine alone and no-one, no deity, no priest can claim dominion over them. The 10 Commandments have two commandments relating to this, one is not to commit adultery and the other not to envy another man's wife. Surely a commandment not to sleep with children could have replaced the envy one?
I look forward to your response, either a rip-roaring Christian epic on how Christian sexual morality can save us from the hell of a permissive and progressive, sexually relaxed secular culture, or a point by point destruction of my claims on the clashes between human nature and Christian sexual repression. And I didn't even start on pornography, prostitution, adultery or divorce!