I am faithful to my spouse and my spouse is faithful to me.

The dramatic version, one decision and one betrayal, hides how the thing actually works. Mostly fidelity is a long run of tiny decisions about where you take your bad days.

Choosing to bring a difficult evening home, to be unimpressive in front of the person who knows you, is a version of it. So is not building a small private life they would be surprised to hear about.

None of this requires suspicion. Faithfulness practised as vigilance is exhausting and doesn't work anyway.

It is simpler as a habit: turning towards the same person, repeatedly, including on the days when turning elsewhere would be easier.