In my current article in JME, some of which has previously appeared on the Betterhumans website, I examine the theory of background conditions - the idea that all human cultures assume the existence of basic conditions in the background of human life that are beyond our choice, though they form the context for our choices.
According to the theory, these background conditions to choice are perceived to be timeless truths. Some of them, in fact, are clearly not true (such as the assumption of male superiority), while some others are true only to a limited extent (the relationship between sex and repoduction), or may not be true in the future if it sees the emergence of a radically different technological context (for example, it may no longer be true that human beings are mortal in the same sense as we always have been, or that work is necessary in the same sense as we've experienced historically). When the background conditions, as understood in a culture, are threatened, whether by practices such as gay sex or by new technologies such as IVF or human cloning, at least some people will feel that what is happening is "unnatural", and will themselves feel threatened.
I'm about 90 per cent convinced that this theory is onto something, though it would be nice to devise some ways that it could be empirically tested. Ordinary social observation certainly provides a lot of data that it can make sense of, but it would be good to have more precise data.
If the theory is true, it might show why there is so much essentially irrational resistance to harmless, or even beneficial, practices and technologies that are stigmatised as somehow "against nature". I certainly don't think it provides a basis for legislatures to ban those practices and technologies, or for rational people to give their support to feelings of disapproval. If we come to the conclusion that the theory of background conditions is true, it can (in part?) explain the "yuck factor", and perhaps the persistence of bad ideas about sex and reproduction, but it should not be interpreted as a justification for what it explains.