1) Hypocrisy - IMO, if a person weren't already a hypocrite, they wouldn't cheat; honest people do not cheat, IMO, and hypocrisy is just a type of dishonesty in my book. Also, WSes are clearly flawed as partners. Since an RA seems to follow d-day pretty closely, the WS is still unhealthy when partners become madhatters. I don't expect many WSes to be honest or effective human beings for many months after d-day.
And then there's the fact that commonly held beliefs about logic, fairness, morality/ethics, whatever do not govern every human being's behavior. In fact, we know that we sometimes have mutually exclusive ways of being moral - dump vs forgive a cheater, for example. Some people deviate from norms, and it's hard to predict who will deviate in what ways.
I can see a WS who cheated to get external validation thinking they 'can't' stay with a partner, if they view the A as external invalidation, for example, and that's only one way a WS can choose to D a BS turned madhatter.
So I understand cheating, wanting R, starting R, and then dumping a partner because the partner cheated.
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2) One primary justification of RAs is that they help BSes feel better about themselves.
To the extent that happens, it's because the RA feeds a need for external validation - and any need for external validation in an adult is unhealthy.
One of the problems - one of the illusions - BSes need to deal with is that they've placed a lot of their self-esteem on the shoulders of their partners - partners who have betrayed them. Whether they realize it or not, they've probably come to feel the external validation they get from their partner is necessary. A BS can't heal without learning they'll be OK without external validation from anyone, much less from their WS. To heal, a BS need to learn to validate themself, even though they may not use that terminology.
BSes need to accept that nothing can change the fact that they've been betrayed. I think - hope - it's fine, even unavoidable, to do some wishful thinking in the aftermath of d-day. But healing happens internally. The BS heals in part by accepting the betrayal, figuring out its impact, and rebuilding themself. The BS is the only person who can do that work.
An RA is a distraction, a wrong turn.
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3) Another justification is to make the WS feel the pain that the BS feels, but that was addressed above. I agree it's impossible to make a WS feel what the BS feels.
4) You state that infidelity by one partner allows the other partner to cheat as well as if it were a fact.
My point is that there are many of us who think the BS's vows remain valid unless the BS chooses D. Maybe a violation of the fidelity clause does kill the whole M contract. But given the amount of disagreement, I think we should treat the idea that the whole contract is dead as an opinion.
I could not disagree more with that opinion. I'm not a lawyer, but even I know enough to add to most contracts a clause to the effect that breaching one part of the contract does not necessarily void the whole contract.
My M contract with my W did not obligate either of us to be perfect. I certainly didn't expect this particularly devastating imperfection from her, but life is risky. That risk has brought a lot of good things into my life, and a lot of pain, too, but that's life. (BTW, I think I've suffered worse traumas than being betrayed, and I know my W has suffered worse trauma than betraying.)
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Generalizations do not help people heal from infidelity. Especially when one turns bits and pieces of ideas into straw men.
To heal, a BS needs to be willing to look inside and deal with the anger, grief, fear, and shame that come with being betrayed. They need to tear themself down and build themself back up from the pieces that want to keep. Generalizations require detachment, which is the enemy of healing.
One doesn't heal from the flu by reading papers abut the flu. One's body has to get dirty fighting the viruses that live inside one's body. That's true for healing from being betrayed, too.
It's not easy, but it's worth the effort.
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This whole discussion stems, IMO, from a misplaced focus on something like fairness. There's no fairness in infidelity. Trying to find some way of making infidelity fair is to attempt an impossible task - and to fail, which may leave the BS feeling even worse.
Trying to justify a revenge A for any reason is wayward thinking. Wayward thinking is not conducive to being a good partner.
The more one gives up the idea that life can be fair, the more one can heal. We really need to accept that there's no fairness in infidelity.