As per my previous post, the two main accounts of the wrongfulness of lying are that what is wrong is insincerity—saying something that one doesn't believe—and that what is wrong is false-telling. Here is another argument against the insincerity account, and hence in favor of the false-telling account. However, I will end by offering yet another account of the wrong in lying, which has significant attractiveness.
Case 1: You are the manager of a motel and you hate the owner. Consequently, you always put up a "no vacancy" sign when you have a vacancy and a "vacancy" sign when you don't, in order to ensure the motel not only does not get much custom, but gains ill-will among travelers. Plainly, you are lying by putting up the sign, and both the false-telling and insincerity accounts condemn your action. You are wronging not only your employer, but also wronging readers of the sign and not just by inconvenience. The action is made wrong by that which makes lying wrong.
Case 2: You are employed by a motel, and you install a computerized sign, linked to the motel's occupancy computer system. You program the computer system to post "no vacancy" when there is a vacancy and otherwise to post "vacancy." You do this with the same motives as the manager in case 1.
I don't think there is a significant moral difference between cases 1 and 2. But in case 2, we cannot analyze the deceitful sign in terms of insincerity. Granted, when the sign says "vacancy", you don't have a belief that there is a vacancy. But even had you programmed the sign to be correct, it might well be the case that you or anyone else would have a belief that there is a vacancy when the sign says "vacancy"—the computer has the information, and maybe no human does. So the fact taht you don't have the belief that there is a vacancy when the sign says "vacancy" is not the reason you've done wrong. Since in case 1, the action is, inter alia, made wrong by that which makes lying wrong, we have to say the same about case 2.
The false-telling account handles case 2 fairly easily. While it is not quite the case that you're asserting that there is or is not a vacancy, you are the person responsible for the correctness of the communication, the one who is being trusted by the reader. (That "vacancy" is not a sentence in standard English is clearly irrelevant, as it means something like that tThere is a vacancy at this establishment.) The false-telling account can be extended to say that one should not make oneself responsible for the correctness of a false communication. The precise sense of "responsible for its correctness" needs some more clarification—an interesting project—but it is plausible that there is a univocal sense of responsibility for correctness present in cases 1 and 2.
I suppose the best bet of the defender of the insincerity account is to modify the account to be conditional: One should not make oneself responsible for a communication's correctness (say, by causing that communication to be produced) when one does not believe that the communication would in fact be correct.
However, it now occurs to me that there is a third account of the wrong in lying, differing from the insincerity and false-telling accounts. This is the intention account. On this account, one is only permitted to make oneself responsible for a communication's correctness when intending that communication to be correct. What is nice about this account is that it handles the counterexamples to the insincerity account, both from this and the previous post. For instance, when one says a German sentence which one believes to express a truth that one does not believe, one is intending to make a correct communication. Likewise, when one makes a prediction that is only going to be true if one speaks (say, the statement: "I am saying the difficult word 'supercilious'"), one is intending to make a correct communication. And finally in the crooked computer programmer case, one is not intending to make oneself responsible for a correct communication.
In fact, the intention account is superior to the false-telling account and to the conditionally-modified insincerity account in the computer programmer case. Suppose the programmer is not crooked, and does things correctly, but knows that sometimes the computer memory will malfunction and misinform the code that controls the sign as to the hotel's vacancy status. On the false-telling account, the programmer has done wrong when such malfunction happens—she has caused a false communication in a way that makes her responsible for its correctness (computers aren't responsible, so all the responsibility flows back to the programmer). On the conditional insincerity account, the programmer knew that sometimes there would be a malfunction, and so she caused a communication that she believed would be incorrect—not specifically, though, but as part of a set of communications. So both of these accounts seem to make the innocent programmer who knows about the likeliness of screwups a wrongdoer.
One might bite the bullet and say that she's inculpably doing wrong, but the intention account is much neater: While she foresees that she will be correctness-responsible for a communication that is false, she does not intend that. She intends the communications to be correct, and regrets the times when they won't be.
My previous post's analogy with adultery then suggests the following moral rule: A married person is only permitted to have sexual relations when the intention is to have sexual relations with the spouse. This has the interesting, and I believe importantly correct, consequence that it is wrong for a married person to have sexual relations in which the fact that the other person is their spouse is deliberationally irrelevant, and wrong in the way adultery is. It would be equivalent to adultery for a married woman to intend to have sex with the nearest attractive person, even if that nearest attractive person were the spouse (unless "attractive" is understood in a way that makes the spousal status be a part of the attraction). This is closely related to John Paul II's remark that it is wrong to lust after one's spouse.
This may, in turn, suggest an argument against pre-marital sex. If pre-marital sex is permissible, then it seems plausible that the couple who are married could intend to have sex with one another in the same way in which they had intended it when they were not married. In other words, it would be permitted for them to intend to have sex without intending marital sex. But on the above account of adultery, it is required that the married couple intend marital sex, and hence it is very plausible that sex without that intention is wrong simpliciter.