A continuing conversation that I've had with quite a number of people involves the idea of sticking your nose into other people's relationship choices. One example that seems to come up pretty regularly is a poly person dating someone who is either cheating on another partner, or in a Don't Ask, Don't Tell. Is it ethical, unethical, or just not your business? Are you "policing" someones relationship by opting out because they aren't being honest with another partner?
The justification I fed myself for the six years I was involved in a DADT was that it wasn't my relationship, not my job to legislate what happened in their relationship with each other, and therefore, perfectly okay for me to have a relationship with this man who I love. I still believe that the first portion of the sentence, right up to "therefore" is about on target. It's the second portion, about it being fine for me to be in that relationship, that I no longer buy into, regardless of the love.
So many things that aren't optimal fall through the cracks based on the apathy of, "It's not my business.". In a relationship structure where each and every person's actions potentially impact those surrounding them, it's worse than foolish to stick your head in the sand and hope for the best.
It isn't my job to tell someone else what to do within their relationship(s), but it is my job to understand what kinds of relationships work for me, and be able to screen for that. Honesty and openness work best for me, so I don't choose to become involved emotionally with those who aren't in that space currently.