Relationship friction, duty and responsibility, loss of control, ego-related conflict

Finding out a spouse has been fooling around with someone else 
Discovering sexy text messages on a partner's phone
Starting a fledgling but promising relationship only to find out the other person is still sleeping with their ex  
The character coming home to find their partner in bed with their best friend
Being dumped by a partner and realizing they've been involved with someone else for a while
The character discovering their significant other, who travels frequently for work, has another family in a different city 

Fielding questions from the kids, who can see something's wrong
Friction around the home (if the couple is still living together)
Having to attend couple's counseling to try to work things out
Ending the relationship with the cheater 
Contracting an inconvenient but treatable sexually transmitted disease due to a partner's infidelity
Seeking to gain the upper hand in a divorce and having to take actions to catch the cheater in the act (by hiring a detective, setting up hidden cameras, monitoring their phone, etc.)
Having to confront the cheating partner about their actions
Becoming the "bad cop" in the relationship (forcing the partner to find new work away from their lover, requiring transparency of financial statements, etc.) to move forward
The character hiding what they know (and the pain that comes with it), hoping the partner will come to their senses and the situation will resolve on its own 
Broken relationships with people who knew about the affair and said nothing