Moral laws

It's not just the moral authorities that we accept. We also accept the authority of moral laws. We do so even if we might grant an exception.

If Laura's husband has lost all interest in sex with her and has done no more than grudgingly go through the motions only when asked these last ten years we can easily imagine that someone might decide that Laura's having an affair was justifiable. You might not agree about this but you can imagine how someone else could talk themselves into it (Laura herself for example).

But we couldn't say that the person who felt an exception should be made didn't respect the moral law that says you should not cheat on your spouse. The mere fact that they need to consider the behaviour of Laura's husband puts them in an entirely different category from someone who believes in open marriage.

This is so because the very possibility of laws depends on there being agreement about how the laws should be applied most of the time. It's a lot like colours. When we talk about "blue" we can do so because most of us agree that certain colours are blue. We do this even though there is royal blue and navy blue which are quite distinct from one another. To be able to talk about blue, as Wittgenstein said, it isn't enough to have a definition of "blue" we must also agree in the judgment that this is blue. And that is true even if we might wonder if maybe blue indigo is more like purple.

So too with adultery. There are variations and maybe even degrees of adultery but there are also a lot of cases wherein everyone will agree that this is adultery. Even the person who thinks that marriages should be open will be able to agree that this set of facts fits the definition (they just won't think there is anything morally wrong with it).