The best couples therapy research proves that we are basically designed to be in deeply emotionally connected and healthy monogamous relationships.
Professional counselors like me were stunned a few years back to see land breaking research that showed that even the most thorny issues distressed couples came to counseling for - chronic conflict, sex problems, parenting disagreements and problems with in-laws etc, spontaneously resolved with 7-10 sessions of laser targeted emotional repair work. And these positive outcomes were still in effect years later at follow up.
That means parenting problems, sex issues and even moderate clinical depression resolved, not by provide special counseling for each kind of problem, but by re-expanding the couples love connection;- by helping them fall more deeply in love again.
It's all about learning to really put our partner's emotional needs first, to hear them and to provide soothing and reassurance in times of stress or insecurity. In contrast to verbal criticisms or emotional shutdowns, this elicits their natural desire to want to be there for us at that profoundly loving and caring level too.
Put simply, 3 or more people can't put each other first!