A form of explanation in ethics

Why are murder and incest wrong? Here are plausible things to say:

1. Murder is wrong because it cuts short a future life of valuable agential activity.
2. Incest is wrong because it leads to genetic defects in offspring.

Now an explanation needs to be true. But murder doesn't always cut short such a future life (e.g., think of the murder of a severely disabled person) and incest doesn't always lead to genetic defects (e.g., suppose the two parties are past the age of childbearing or are of the same sex). So the explanations in (1) and (2) must be understood as making Aristotelian categorical claims, like "Sheep have four legs", claims that are true in normal or paradigm cases. On the other hand, the claims on the left side, that murder and incest are wrong, are meant to be true in all cases. Thus, (1) and (2) can be made more explicit as:

3. Murder is always wrong because normally it cuts short a future life of valuable agential activity.
4. Incest is always wrong because normally it leads to genetic defects in offspring.

Now, it seems that there is something deeply fishy about (3) and (4). How can the fact that normally murder and incest result in certain grave harms explain the fact that they are always wrong? Yet, murder and incest are always wrong, and the harms cited seem to have something to do with their wrongness. One could say that the harms cited explain only the wrongness of normal cases of murder and incest, and other cases are wrong due to other harms. But I do not think this is desirable. Similar worries are likely to apply to the other harms. Maybe one can find a set of such harms such that every possible case of murder or incest is made wrong by something in the set, but that is not so likely. One could, I suppose, abandon the claims that murder and incest are always wrong, but that's a serious moral mistake.

But on a number of ethical theories the explanations in (3) and (4) are perfectly fine.

Rule utilitarianism: Here the point is clear: an act can be wrong precisely because most of the time it is seriously harmful.

Divine command: Because the acts result in such terrible harms in most cases, God wants us to stay far away from these acts and wisely forbids them to us in all cases. So the fact that normally great harms result explains God's universal prohibition, which in turn grounds the universal wrogness.

Natural law: The natures of things support their flourishing as individuals and as a kind. That an action type is normally harmful to the individual or the kind makes it likely that the action type is unnatural, and hence wrong. This explanation becomes more satisfactory, I think, on a theistic natural law theory. For then we can explain why it is that the natures of things support their flourishing. On a view on which God designs natures, one can say that God is unlikely to design a nature that fails to support the flourishing of an individual or kind. On a view on which God finds (in his mind) natures and then decides which natures should be exemplified in creatures, one can say that God is unlikely to choose to exemplify natures that do not support the flourishing of the individuals or kinds. On a non-theistic natural law theory, it may be a bit more puzzling why the natures of things support their flourishing. Maybe an evolutionary explanation can be given, though.

Notice an interesting difference. In an appropriate rule utilitarianism the harm facts might ground the wrongness facts. In the divine command and natural law cases, they don't ground the wrongness facts, but explain them in a less direct way.

In all of these cases, the same line of thought that leads to the explanations allows for the following argument:

5. Action type A normally produces great harms.
6. So, A is always wrong.

On an appropriate rule utilitarianism, this could be a deductively valid argument. But on the other theories, it is only a defeasible inference.