Because, obviously, you can't know that you won't. Don't you think Larry Craig thought he wouldn't be caught? A little hanky panky in the men's room--who needs to know? What's the harm?
But then we've moved away from the terms set out in the OP, namely a consequentialist understanding of ethics. What's wrong with the tryst is not that it causes harm. It doesn't.
I would disagree. There is no way to lie without becoming a liar. There are two people at the end of every action. You can't lie without lying and knowing you're lying and having to live with that knowledge. Harm.
Further, you have deprived yourself of one of the best things life has to offer--to be known, truly and honestly known, and still loved. The only way to get this gift is to be honest. Again, no shortcut. As soon as you lie, you've deprived yourself of this. Harm.
I was watching a documentary the other day where several people were interviewed who had cheated on their spouses using an adultery Web site AshleyMadison.com. (
Here's an article about it. They paid some firm to arrange alibis and all the other rigamarole to make sure their spouses were entirely in the dark. And if what these people say is true, the scam worked. The wife (typically but not exclusively) is none the wiser, and some of them swear on a stack of bibles that their relationships have improved as a result. They have various stories to say why that might be, but most of them devolve to learning through the affair just how much they love their spouses. We might want to scoff at such stories, but there is no reason to disbelieve them.
It's important to also point out that just because these stories are true, we should not give up on the conventional wisdom. All the truth of these stories entails is that it is possible to have an affair with no harm done. And if THAT's possible, then our consequentialist principle should be rejected in favor of something else.
Anecdotes are not data, especially self-serving anecdotes. Again, research shows that the most important thing in our own happiness is the quality of our relationships with those we love. Another important thing is our own estimation of the quality of whatever we are doing. If you can say to yourself that you're treating your wife well, that she feels loved, and that you, the real you, not the fake you, are loved by her, you're a very lucky, very happy man. (or in my case, woman.)
Didn't your mama never tell you: Oh what a tangled web we weave, when first we practice to deceive?
For evolutionary reasons, we humans experience everything we do to others and empathize with its effects. You can't stop yourself, unless, as I say, you're a psychopath. There's a reason the golden rule asks us to treat others as we would be treated; it's the path to happiness. When you are cheating on your wife, you cannot stop your awareness that you are doing so, and that you would not want to be treated the same way. You know, and can't help but know, you are treating your mate, your partner, badly. And you can't help but feel bad about that, or at least not as good as you might feel.
It's one of those hedonism vs. wisdom things. It might sound appealing, but in the long run the wise choice leads to your greater happiness.
Basically, your antidote to harm is lying. So that brings up an interesting ethical question. Why do we all feel, intuitively, that lying is wrong?
The question is whether it is wrong to do so if you can arrange it so she can't ever know about it. If one can, it's apparently permissable (based on the ethic of "You can do it but only if it doesn't cause any harm").
1. You can't guarantee that. 2. More importantly, you will know about it.
Hey, we're talking about a one-off here not a lifestyle.
That semi-facetious point aside, let me just say that this piece of homely, stock wisdom is entirely trustworthy. Nevertheless, it is possible (and that's all I need for my argument) to have a one-off tryst with no harm done. And if that's so, the consequentialist principle is refuted.
You have failed to show that no harm is done.
No doubt, there are those who cheat and get caught. Is the problem the cheating or getting caught?
Both. How do you think Ted Haggard felt before he got caught?
A few disclaimers. The only thing my story needs is the reduction of harm to zero or to such a small level that it's hardly worth considering. Doing this does not require the sort of psychology you attribute to my hypothetical character. No plan is perfect, so no, he can't be 100% sure he won't get caught, but the chances are, ex hypothesis, minimal, so he doesn't particularly worry. And of course the lifetime of trust is worth more than the one night stand. That explains the care with which he ensures his wife won't find out. If his wife doesn't find out, there's no loss of trust or intimacy.
This is where I disagree. By intimacy I'm referring to a relationships based on honesty, on reality. An intimate relationship is one in which I am real, authentic, what I really am, and also accepted or even loved. You've cheated yourself of that.
And of course, there's no need to assume that the intimacy of the relationship will improve. I only submit that it's possible, at least if we take the stories of some cheaters at face value (something we have no reason not to do). And all I've said is that if this is true, the consequentialist ethic gets turned on its head so that cheating may be obligatory. The fact you find this result counterintuitive (as I do) shows just that you don't seriously hold to a consequentialist theory of ethics.
Not the intimacy I'm talking about.
btw, note that the intimacy I'm talking about is NOT harmed by a polyamorous relationship, which is based on honesty and trust.