I don't understand how you would ever build a level of trust with that person. If they can do it to someone else they can just as easily do it to you. All the things they are saying to you...you're different/special, I tell you everything etc. etc. they are probably saying to the person they are keeping in the dark.
Playing the Game of Life with Monopoly rules.
Monogamy might just be in my genes
Poly Events All Over