that we all need someone; a life-partner that we commit to help care for, trust, communicate honestly with, etc. We benefit greatly from the strength and security that total commitment provides. So many studies clearly show that we have less stress, live longer, stay mentally sharper - so many benefits.
While I also believe that we all need the solace, comfort, joy and ecstasy of physical touch and coupling... I do not think this necessarily needs to come from that one life partner. Yes, in some cases that match is so good that it does; in others not so much. When too far out of balance....
Too many couples ignore intimacy needs and this erodes the sense of open and honest communication, trust and caring between them - the very things that form the strongest elements of their pair bond. I say better to share the physical openly if needed, than to sacrifice the more precious elements of your most important relationship. I see physical sharing as much more likely a way to preserve the essential elements of a "monogamous" relationship than attempting to maintain physical monogamy as well. In fact, I think it can add much to a relationship where the physical intimacy is failing - and believe me, if it is failing for one it is failing for both in some way. Find solutions, not blame?
I say "YES" to monogamy, and "YES" to faithfulness - if we mean by faithful that our trust and commitment to one another remains unbroken, regardless of the means we agree to in order to meet physical needs.