Distance in a romantic relationship can be dangerous. Unmet personal needs can create unhappiness, hard-to-control yearnings, and more. Should the right temptation come along, a once-committed partner might make a decision that sets them on a collision course with shame, guilt, and regret. Whether it's a one-night stand or a longer affair, a character who cheats will do anything to keep it secret, not only to avoid hurting their partner but also to save themselves from the personal, financial, and reputational repercussions of such a betrayal. 

Betrayals and Violation of Trust (primary), Family and Relationships, Illegal or Immoral Acts

DESIRING A FORBIDDEN RELATIONSHIP, HIDING A SEXUAL RELATIONSHIP, NO LONGER LOVING A PARTNER

ABANDONMENT, BECOMING WHAT ONE HATES, BEING JUDGED, BEING SEPARATED FROM LOVED ONES, CONFLICT, HUMILIATION, ISOLATION, LETTING OTHERS DOWN, LOSING THE RESPECT OF OTHERS

Having to lie repeatedly to their partner, deepening their shame and guilt
Being stuck in limbo; wanting to move on but being afraid of the consequences
Expending great time and energy keeping their two lives separate
Feeling immense guilt, especially if there are children involved 
Constantly worrying they're going to get caught
Not being able to fully commit to either relationship
Struggling with anxiety, hypertension, and other signs of prolonged stress
No longer finding joy in family events; always wanting to be elsewhere

Saying things to their spouse that they don't feel
Buying extravagant gifts or flowers to feign interest
Overcompensating (offering to help in ways they normally wouldn't)
Being extra cheerful and upbeat all the time
Being overly attentive to the children
Spending more time at work
Fabricating meetings, trips, and late working hours so they can be with the other person
Using cash when they're with the other person
Frequently deleting emails and text threads
Asking a friend to back them up or provide an alibi
Openly criticizing infidelity; making big claims about valuing loyalty

Their spouse finding a “burner” cellphone
Being secretive about their phone (changing the password, not wanting other people looking at it, etc.)
The partner discovering a second bank account or credit card
The character taking up a new sport or hobby that doesn't make sense for them
Suddenly acquiring new “friends” the spouse doesn't know and never gets to meet
Frequently having to work late at night or over weekends
The history on their phone or computer always being empty (erased)
Texting a lot more than usual
Contradicting themselves 
Showing decreased interest in their spouse
Being caught in a lie
Their clothes smelling like an unusual soap or perfume
Forgetting to remove incriminating objects from their pockets that come out in the laundry (a hotel key card, a receipt for jewelry or flowers, a personal note from the lover, etc.)
Their demeanor changing when they're around their lover

Vacationing or attending a work conference with the other party  
Going to an event that both the spouse and lover are attending 
The lover being someone the partner knows and spends time with occasionally
Running into the lover unexpectedly while with the spouse
The lover threatening to reveal everything
A teenage or adult child becoming suspicious and confronting the character
Being seen with the lover by a close friend of the family
A family crisis that requires more of the character's time and attention
Having to attend couple's therapy
Being confronted directly by the spouse 

Children growing up and moving out, leaving the character alone with their spouse
Discovering that their partner is also having an affair
Realizing they're in love with the other person and that life is too short to stay in an unhappy marriage
Realizing the importance of their marriage and wanting to make it work
Almost getting caught, and realizing the risk is too great
Having to relocate far from the lover
A chance to pursue a personal or professional dream that requires increased focus and the removal of distractions

The marriage ending
Children wanting nothing to do with the character
Becoming the source of gossip among friends or coworkers
Being cut off by extended family
The character having to live with the knowledge that they deeply hurt their partner
Being free to pursue the relationship with their lover
Living openly and honestly
Realizing how important their marriage is and deciding to fight for it