It is complicated!!! I think cheating is a direct action that comes from a stage of loneliness and absence of all three parties. All vertices of the triangle have something to do to make this happen. So to stop it one of the parties have to take a deep look if that type of relationship is worth it or not.
Relationships are complicated imagine if you bring one person in it!!! It's a chaos !!!
So to avoid that, I would said is always keep the eye contact, the touching, hugs, sometimes couple go to work and the "kiss to each other" and if you ask them, what is your husband or wife wearing today?, they don't know...shocking true!!!
Keep the interest for what they were together, have at least one thing in common to enjoy it and other any many different to share with. Keep the flame of love burning is not an easy task by it is a great challenge to take.