It doesn't matter what kind of relationship structure you're involved in so long as there is trust and communication.

I have a friend with four partners who is very happy. Their partners have different partners and they are all faithful within the framework of their community. There is nothing abnormal about the interactions and they still encounter problems that a normal 'monogamous' couple would experience. (Including occasionally using a therapist to help relationship problems)

Humans are serial monogamous because social modeling has entrusted most with the idea that its important to be relationships with one individual and no one else. I believed in this construct for a long time until I met individuals who break the mold.

It doesn't excuse the fact that a lot of people just grow up learning mal-adaptive behaviors that are detrimental to a healthy relationship and then they grow older and repeat those behaviors without knowing any better.