Let A be the set of all the sexual activities that our society takes to be wrong. For instance, A will include rape, voyeurism, adultery, bestiality, incest, polygamy, sex involving animals, sex betweeen adults and youth, etc. Let B be the set of all the sexual activities that our society takes to be permissible. For instance, B will include marital sex, contracepted marital sex, premarital sex, etc.
Now there is no plausible philosophical comprehensive unified explanatory theory of sexuality that rules all the actions in A wrong and all the actions in B permissible. There are comprehensive unified explanatory theories of sexuality that rule all the actions in A wrong but that also rule some of the actions in B wrong—the various Catholic theories do this—as well as ruling same-sex sexual activity wrong (which isn't in B at this point). There are also comprehensive unified explanatory theories of sexuality that rule all the actions in B permissible but that also rule a number of the actions in A permissible—for instance, theories that treat sex in a basically consequentialist way do this.
If this is right, then unless a completely new kind of theory can be found, it looks like we need to choose between three broad options:
- Take all the actions in A to be forbidden, and revise social opinion on a number of the actions in B.
- Take all the actions in B to be permissible, and revise social opinion on a number of the actions in A.
- Revise social opinion on some but not all of the actions in A and on some but not all of the actions in B.
It is an interesting question how one would choose between (1) and (2) if one had to. I would, and do, choose (1) on the grounds that typically we can be more confident of prohibitions than of permissibles, moral progress being more a matter of discovery of new prohibitions (e.g., against slavery, against duelling, against most cases of the death penalty) than of discovery of new permissions.
It is somewhat easier to overlook a prohibition that is there than to imagine a prohibition that isn't there. An action is permissible if and only if there are no decisive moral reasons against the action. Generally speaking, we are more likely to err by not noticing something that is there than by "noticing" something that isn't there: overlooking is more common than hallucination. If this is true in the moral sphere, then we are more likely to overlook a decisive moral reason against an action than to "notice" a decisive moral reason that isn't there. After all, some kinds of moral reason require significant training in virtue for us to come to be cognizant of them.
If this is right, then prima facie we should prefer (1) to (2): we should be more confident of the prohibition of the actions in A than of the permission of the actions in B.
It would be interesting to study empirically how people's opinions fall on the question whether to trust intuitions of impermissibility or to trust intuitions of permissibility. It may be domain-specific. Thus a stereotypical conservative might take intuitions of impermissibility to be more reliable than intuitions of permissibility when it comes to matters of sex, but when it comes to matters of private property may take intuitions of permissibility to be more reliable, and a liberal might have the opposite. But I think one should in general tend to go with intuitions of impermissibility.