I take it that you're asking me, moby? I'm not quite sure what you're getting at?
There's no doubt that almost the entirety of the blame lies with him. It depends on how the wronged party views it, I suppose, but her views will not always be rational...so, yeah, almost the entirety of the blame lies with him. He's going to find someone to do it with, whether that person is aware of his marital status or not, he may even pay for sex.
I still don't see why you would take part in such an activity though. It might not be your fault or even your problem (although it could be, you never know how that wife might react if she finds out who you are!
) but if you know that it will lead to a heartache, why do it? You can find some hassle, guilt-free tail elsewhere!
Also, as I said, if you did know the situation, would you really respect the guy? If not, why sleep with someone you don't respect?