"Blame should be placed always with the actor."
I wouldn't have put it the way the GP did, but yeah, "blame," if we have to assign it, probably does always land at least partly on both partners in a relationship. PG was proposing a hypothetical in "outrageous" terms as a counterexample, and arguing about the specifics of the counterexample is probably pointless. We aren't going to establish what the specifics were in the situation because it doesn't exist.
By "always blaming the actor" you also blame the woman who leaves an abusive man. You also blame the man who leaves his wife because she had an affair (though she doesn't want the marriage to end).
You're attaching a heavy moral burden to the notion of a failed marriage, and I think where the "Both partners are always responsible for a divorce" counselor is coming from is not one of moralistic condemnation. You're lumping an affair in with rape and natural disaster. At best, that's hyperbole, at worst you're trying to impose your moral code of fidelity on other people's children.
In a world where about a third of both men and women "cheat" on their spouses, I think it's hypocritical of society to be universally and unilaterally condemnative of the behavior. If we're talking about the culpability in breaking a promise of fidelity, we also need (in the context of our hypothetical affair situation) to talk about the culpability involved in the other promises of marriage. Was she loving, honoring and obeying him? Was he? I don't accept that these thorny and extremely poorly-defined questions can possibly come down to a simple equation of blame.
You might accuse me of casuistry, and I can see that. But as far as I'm concerned you can't possibly come up with some equation of "blame" for a failed marriage whose definition is so precise and so universal that there is any circumstance where you can objectively and unambiguously assign it to only one partner.
A better example lie, in the context of a divorce, might have been "children are always blameless." It's obnoxious to most people to contemplate the idea that children in a family might contribute to a divorce, but this is undoubtedly sometimes true.