The two cornerstones of a healthy relationship are Honesty and Trust.

Can you imagine someone saying, "I have a great marriage! I just don't trust my wife?"

Or, "He really loves me, but he is not honest with me."

Of course, we would instantly know that any reasonable relationship must have, at its very foundation, honesty and trust. 
The relationship may have other powerful connections, or pieces that feel terrific, but without these two critical components, it will not be healthy: it may survive, but it won't be healthy.

If you have prior infidelities, they must be processed. There simply isn't the possibility that ignoring the past or brushing it under the carpet will make the relationship better, or even return it to its set point of comfort that existed pre-infidelity.

It's important to remember that any relationship that has persevered beyond any period of time (say, 5 years or more) will experience psychic injury to some extent. 

Human beings inherently let each other down - we will be disappointed by our partner at some point.

Processing a psychic injury can be done. A skilled clinician, with a motivated couple, can provide the space of discussion, actually hearing one another, facilitate meaningful amends (beyond "I'm sorry"), and allow for forgiveness, with room for the relationship to thrive. 

Most couples want to preserve their relationship (keeping the family together, maintaining the life that they have created) - but a couple in distress does not want more of the same.
"Would you like a new and improved relationship, with the same partner?"