I wasn't always poly...
For me, poly was an evolution, a logical choice and one brought about by personal growth. I have no regrets in how I came to be poly, but I also would not have objected to having a conversation with a poly person back when I was mono, someone trying to show me the logic behind the philosophy. I believe that each person will grow in whatever direction they need to at the time they need to, so it is hard to tell someone something they don't already know, but still, I can't help but wonder.
I have definitely affected at least one person by introducing them to a poly way of life before they would have come to it on their own. I have also introduced people to poly with whom it did not work, but even in those instances we went our separate ways with new understandings.
As for married people or people in otherwise mono relationships, I say if the attraction is there, perhaps they aren't as mono as they think they are. I don't want to be a part of a deception, so if a monogamously engaged potential partner wants to pursue something with me, it needs to be above board. I have even been in a potentially romantic relationship where I was "putting on the brakes" for the sake of her partner who was having a hard time dealing with the idea of sharing his partner's affection with me. I had a "date" with him that lasted longer than my date with his partner!
Often he had been often bound with fetters and chains, and the chains had been rent asunder by him, and the fetters broken in pieces: neither could any man tame him...and he asked him, What is thy name? And he answered, saying, My name is Legion: for we are many.