Earlier quoted context omitted.
That doesn't sound like a healthy relationship.
For what definition of "healthy"? Few marriages are perfect, and many are quite broken in many respects. Stress drives people to do all sorts of things they might later regret. But if an affair is unhealthy, by that definition literally most marriages are.
I would be inclined to say that yes an affair would be a characteristic of an unhealthy marriage; at least by western standards. However I'm not sure you are correct in stating that most marriages have one or both partners being unfaithful. [0]
[0] https://en.wikipedia.org/wiki/Infidelity#Infidelity_and_gend...