i myself am poly, my wife is mono by nature, and yes she is seeing someone, but has a hard time with me doing the same for years now, but it is by my complete honesty, and that she is now ok with me with others. mind you now it took me several years of telling her everything, and also not continueing a relationship if they chose not to talk to her as far as there wishes go. she sees that as a respect thing and i fully agree, personally i dont see it is neccary for to tell me everything, or the other to as well, i only want know if they are pursuing a relationship, and if it goes as far as intamacy i want them both to come talk to me first.
i have had several, but didnt get past first meeting do to non respect on there part. not sure why but no one ever wants to talk to my wife to let her know of there wishes. we not asking to be friends just on a talking level. to respect each other.
so yes for it to work at all you must communicate, talk to all ionvolved, and be honest. otherwise you might as well be with one partner and cheat on them.