Strange that you mention this point as well. It was never part of the original discussion but one of Lin's interpretation of the points the husband may have made. But the husband's enrage was more about his wife being able to accept me and even defend me when they argued, which accounts to his main problem being me/his wife, sharing this notion that it is possible to love more than one and live up to it. (Even though this is nothing my friend would like to explore, her husband was jumping into his own mess of assumptions there.) It wasn't really any judgment about Lin being a homewrecker. I guess that is somethign that automatically comes to mind in the 'third person's position' joining a pre-existing couple.
Facts: 30, female, bi, v-type relationship with Sward (husband, straight, mono) and Lin (boyfriend, straight, mono), poly-fi and co-primary.