[infidelity] do single email accounts and open passwords help

There's a part of one of my books where the husband becomes suspicious of his wife, not in infidelity but in disloyalty to the group they've set up. A key employee comes to him to voice her suspicions about a money transfer, which raises issues in itself but still ... :

‘Another thing. I noticed now that you accessed her account without a password.’

‘Our accounts are open to each other. We use a password to enter the computer.’

‘Very touching. Your idea or hers?’

‘Mine.’ ‘And she went along with it?’

‘Yes.’

‘Wouldn’t a woman who knows that £5000 is coming her way do something to create a password? Or alternatively, wouldn’t she start up a new account in another name?’

‘What are you saying?’

‘She either doesn’t know about this money, she’s naïve or else she’s an amazingly cool operator.’

Running a single email is what some couples are doing and it's a double-edged sword:

James Furrow, a professor of marital and family therapy at Fuller Theological Seminary, an evangelical school in Pasadena, Calif., said sharing an account can be helpful if the goal is promoting openness.

But he said the practice can hurt a relationship if it's meant "as an act of deterrence." "We can take steps to manage our behavior, but then the problem with that is it begins to become the emphasis rather than the trust of giving the other the benefit of the doubt," Furrow said. "What you end up with is the doubt."

He has a point about trust, the central issue in many partnerships. A single email account can deter the would-be philanderer from outside but does nothing to help trust. And just how much contact is permissible before it becomes cheating? It's a sad thing but any sort of contact between a man and a woman, however innocuous, is going to cause slurs and suspicion and if he or she is married - that's a double-whammy.

We're not talking here about the partner who already wants to cheat or to have some light relief, we're not talking about an unattached freewheeler who can't commit and wants to play the field well into his or her 30s. We're talking about a contact where one or both parties have partners already and both want to do the right thing. If the groundrules are laid down, it deters the crossing of any lines and allows the contact to be maintained without suspicion.

I'm not sure about joint emails but each to his or her own, of course. Passwords then become the issue. Do both partners have them or not? Is there any innocent reason to have separate passwords? Are there some things which are said which, while not cheating per se, would not be what one partner would want known by the other?

Or is that cheating?

Can each partner in a relationship maintain other contacts of the opposite gender or does that come under the umbrella "cheating", by definition? Should the method be to include the other's partner in the trialogue or quadralogue?

How to resolve this thing?