I hear y'all. I certainly agree with what you are saying:
It would have been best had my WW not had the affairs.
Second best would have been that she told me about them without my having discovered them.
Third best is me discovering them.
Fourth best is someone else letting me know about them.
Worst would have been not knowing and continuing to live (unknowingly) in the shadow of my WW's fraud--unable, as refuz points out above, to act on my own behalf because I lack the information necessary to do so in a meaningful sense.
So, since I agree that worst would be to live in that fraudulent situation, the question is certainly NOT whether OBS should know, but whether I should tell her.
As I said at the outset, it certainly does seem that, if I stand on the conviction that she should know, then I am under the obligation to tell--or if not under the obligation to tell, at least I wouldn't necessarily be doing anything wrong by telling. But, if on the other hand, in telling I have reason to believe I'm going to cause additional harm to myself or to others, then there's a reason to be cautious about deploying that general principle as a rationale for telling.
There's a somewhat famous thought puzzle in some of Plato's dialogs that seems like it might be useful in clarifying what seems to me to be more complicated than a question about whether OBS should know about the affair (I take it as a given that she should). In Plato, the thought puzzle generally takes a form something like the following: if someone lends me a weapon and asks for it back, I'm generally under the obligation to give it back since it is not my property. But if the lender asks for the weapon back and I have good reason to think that she or he is intending at that moment to put it to use it for a purpose that will do themselves harm or that will harm an innocent person, most of us recognize that the obligation to give what is owed is superceded by an obligation not to provide the means for some other, greater harm.
I am not convinced that the situation in this case is genuinely analagous to Plato's example. For one thing, I don't have anything like good evidence regarding the likely response of OBS... I don't know her at all, don't know what kind of a person she is, don't know how she's likely to respond, don't know what damage she might be inclined to do to herself or others. I only have WW's take on this, and WW is, by definition, not a reliable narrator where this matter is concerned. But, nevertheless, WW is my only source of information about this, and she says there is a reason to be concerned.
Second, let's say for argument's sake that I accept WW's reading of the situation. For the analogy to Plato's example to hold, it would need to be fairly clear that a likely harm consequent upon my telling is greater than the likely harm of not telling. But the analogy also seems to break down here. I'm not at all certain about the harm of not-telling, but I am pretty certain of the kinds of harms that come with not telling. Every one of us here are aware of that harm.
So... your comments are helpful, to be sure, since they are helping me think through the complex sets of obligations here. Among other things, the point that you each have made (more or less explicitly in different posts above), that, by not telling, I'm only delaying and possibly aggravating an eventual trauma for the OBS, is one I hadn't been attending to. What happens, for instance, if I don't tell and she finds out anyway? What harm would I have prevented, if any, by delaying?
I've promised WW to at least wait until we've had an opportunity to discuss this with our MC this Friday to see what perspective we can find there. As I said, this particular affair (leaving aside a few sporadic flirtatious online interactions over the last three years... I know, I know... no need to say it: those are really a perpetuation of the affair, but those sporadic interactions took place in the midst of a second, much more serious second affair, so you can appreciate that I'm not rugsweeping where those flirtations are concerned) seems to have ended in 2010, so I can afford a little bit of time to think through this before acting.
That it might seem I'm stalling here in no way diminishes my gratitude for your input... if anything, it simply provides a little bit more time for me to receive additional input here.
(Still no additional perspectives from WSs, BTW. I really would welcome that point of view if any of you happen to read this.)