You should cleave to your mate, which means not just sexually but every which way.  There could be too much togetherness.  There always has to be some space between both of you.  This separateness expands and contracts.  That is known as communication.

Each partner needs a sense of perspective about themselves and the other.  An outside friend always helps.  Someone you feel safe with and at the same time does not threaten your relationship.  Particularly in relationships you have with the opposite sex you have to be very careful.  Engage in communication under very safe conditions that do not unnecessarily threaten your partner.

And your situation becomes very difficult when every relationship threatens your partner.  It is a balancing act.  You always need to maintain your integrity.  Loyalty to the higher standard (truth, God, and if you want to call it the higher power) is always essential.

You want to remain loyal to your mate but at the same time it is important to keep your sense of perspective.  The only way that can be done is by having outside friends.  Those relationships also prevent you from becoming stale to your partner.  It is a paradox:  you must cleave to your mate but at the same time keep some distance (or proper perspective about each other).  Both partners are enriched by outside friends.

And each time your contact ends with your friend you have something more to share with your partner–maybe a slightly different take on something.  It is, always, as I said a balancing act.  You are always enriched by maintaining your perspective about each other.  The last thing you want to do is have your relationship to become inbred (and stale).  Then your view of each other becomes magnified (instead of realistic).