We agree with BBarnsworth. If both arrive at a point in ENM that although they are life-partners in every sense of the term, can freely talk/express their feelings, emotions, desires to each other as well as others in their relationship having alternative/additional relationships can be very healthy and rewarding.
We have experienced this in our transition through the first six/seven years of the ENM Lifestyle. We both have had others, couples, solo’s from couples as well as singles that we enjoyed in-depth romantic/intimate relationships with. Most lasting years. They only ended due to life changes, job transfers, out of state family obligations. Both socially as well as intimate. It made a huge difference in our personal relationship as well as social/public face.
Her longest relationship was eight years. We traveled and socialized as a Triad. Family and friends socialized with us this way. I’m sure they all had their suspicions but none ever asked. I had three longer term relationships one lasting 6 years. It didn’t ever get messy or emotionally challenging. We both lost our longer term relationships due to life changing issues. With hers he had to move due to family issues and mine she move for an education opportunity.
When all are emotionally comfortable and of the same mindset it is very natural. In our opinion for this to last and grow naturally it needs to develop into an even/level polyamory relationship with partners being equal with each other. We have learned this by having close friends that live their lives in this type of relationship. One very close friend has three husbands and children (it is accepted, not interested in DNA testing) from all three. Neither of them are in the lifestyle/swinging. And all five children call all three Dad.
So, can it work? Yes of course. Does it need to or does it have to? Absolutely not.