It is an entirely relevant comparison. Rape still happens in marriages, as does physical violence. However we know better than to blame the victim and say that he, she or the relationship itself caused it, or that anything was an excusable catalyst for it.
many moons ago on this thread Charbon wrote:
Relationship dissatisfaction and a desire to end a relationship might well be caused by marital problems
Which is presumably why MBP, your friend doesn't blame herself for her husband's choice to have an affair but takes responsibility for her part in the problems that beset her marriage. Since you appear to agree with your friend's take on this, I struggle to see where there is disagreement on your part to what posters (including Charbon) have been saying.
This is precisely the distinction posters have been making; that people have joint responsibility for their relationship but no responsibility for the unilateral and secret decisions made by the other half of the partnership. Their actions might have caused unhappiness and dissatisfaction in their partner either entirely or partially, but if they are not equal partners in the solution and aren't even privy to it, those actions cannot be held to be a cause of a solution chosen by only one of them and kept secret.
As for this:
I accept there are men who just want to fuck women whatever- even if their marriages are ok
This is ridiculously simplistic and sexist. Not all affairs when the marriage is okay are just about 'fucking' and those that are, engage women as well as men.