I can only really speak from one angle of a relationship on this issue; Mr. A had expressed to TP several times that he feels (has felt) like the other man and that he was doing something inherently wrong....now from my perspective there was very actionable things I can do to alleviate this feeling in him....I've talked to him, reached out to him quite honestly to tell him that I am comfortable and not feeling like I am losing time with TP...but at a certain point I had to realize that there was only so much I could do to make him feel welcome.
A lot of his feelings were coming from his upbringing and background. At a certain point you've done all you can conceivably do to help, and there has to be movement on the other end. While you do have to be welcoming, there has to be some reciprocation on the other side to get over this....it's not all you.
Polyamory is wrong! It's Multiamory or Polyphilia. Mixing Greek and Latin roots? That's wrong.