Originally Posted by MMMark
Harriet loves Arthur, but finds that he tends to interrupt her during conversations. She allows him to do this, preferring not to be confrontational. Harriet meets Andrew one day and these two also form a relationship. Andrew is a superb conversationalist and listener, and he never interrupts Harriet. She finally sees what good conversation really is, and the next time Arthur interrupts her, she admonishes him. She is no longer willing to put up with Arthur's rude interruptions. So, Harriet's relationship with Andrew has affected her relationship with Arthur.
Seriously, poly relationships are simply relationships. Place your example in the context of friendships. Harriet and Arthur are friends because they like each other's company. Harriet and Andrew become friends. Do you really think Harriet is going to go all schoolmarm on Arthur because Andrew approaches conversation in a different fashion?
I suspect not. Harriet is Arthur's lover despite his conversational habits. She likely has plenty of friends (and this is directly applicable) who don't interrupt during conversations and that hasn't affected her relationship with Arthur. Her getting involved with Andrew is absolutely no different than her being friends with Alex (who also does not interrupt) when it comes to conversational styles.
The only way your examples--and underlying question--make sense is if one posits that Harriet has no other relationships of any sort and the arrival of Andrew is earthshaking because it provides her with sudden understanding that people can relate in a wholly different fashion. As she *already* has friends who don't interrupt, it's obvious that Arthur's tendency to interrupt isn't much of a hindrance in that relationship, so getting involved with Andrew isn't likely to change much.