The converse accident is a logical inductive fallacy. It’s where an exception to a generalization is applied to cases where the generalization should apply.
In the debate over gay marriage, exceptions, such as infertility and the use of birth control, are used to demonstrate that the pro-creative component of marriage is a non-essential good. Apparently, as with mathematics, all it takes is one exception and the generalization collapses like a house of cards. Fortunately, life is not a mathematic problem. Our legal system governs choices for which there is an opportunity cost that, to some extent, we all have to bear. There is no mathematical certainty in the social sciences. There are always exceptions that a general rule can accommodate, as another example of a converse accident shows:
If terminally-ill patients are permitted to use heroin, it proves there is an exception to the notion that there is a general rule that society has forbidden and will always forbid heroin use. Historically, in many stable societies, the psychedelic forms part of an important religious experience that responsible citizens participate in, e.g. the Delphic oracle, peyote use in Native American rituals. These exceptions prove that the use of narcotic drugs has not only been permitted, but also endorsed in the interest of society.
Consequently, there is no general rule that heroin use is either generally forbidden, nor is it proven to be a consistent detriment to society. In fact, its use by terminally-ill patients proves that it can be permitted in a controlled manner without detriment to society. It therefore follows that all adults should be permitted to use heroin in controlled quantities without the threat of legal sanctions.
Of course, when you highlight the distinction between terminally-ill patients and the rest of society, the proponents of this view claim that you are begging the question: reasserting that heroin use is normatively prohibited outside of the cases that they cite, when the prohibition on the use of heroin is the very premise in question. In fact, you are not referring to the general rule to prove the premise, you are merely showing that the terminally-ill are still exceptions to the majority.
Nevertheless, as one writer put it: ‘The truth of a general rule, on the other hand, leaves plenty of room for exceptional cases, and applying it to any of them is fallacious.’
The real question is whether any of these exceptions are valid counter-arguments that justify dismissing the limitations of general rule and incorporating the converse accident as part of the principle instead. Do the exceptions prove that legalising heroin is a good idea? Is society decidedly better off by making heroin legal?
Ultimately, humans are not laboratory rats. The law may decide that the potential consequences of permitting controlled adult experimentation with heroin are too costly. Society can accept the preponderance of evidence based on the numerous cases of harmful heroin addiction; cases that prove that generally legalising heroin would be a bad idea.