There is another issue that the writer frequently sees in couple relationships. This is where one partner can not trust the other partner.
Usually, there are unspoken boundaries in a grey zone on what is appropriate non sexually in the coupleship.
Some things may be more obvious. Do not have intimate sex or an intimate sexual relationship unless there is an agreement to it in an open relationship.
The grey area is who can you talk to, what is an appropriate conversation with the opposite sex or another person in the relationship. How can I look at others sexually, and many other questions? These issues need conversations and boundaries for the couple.
Underline this type of issue could be a sexual addiction, sexual acting out, or illegal sexual activity. It may be what the partner is seeing is just the tip of the iceberg. For more information on sexual addiction visit this counselling page.
This is an issue that usually needs therapeutic intervention.