Reading back on the thread, I see your first response to her as condemnation and judgment. Then you wonder why she tells you to go piss off? Dude! Do you even realise how douchy you sound? Then someone calls you on it, you get all wide-eyed and innocent, pretending you're the victim. Those are some serious blinders you have there.
Poly: Common short form for polyamorous. The chosen lifestyle where one entertains multiple ethical relationships at once. There are many people in the world that are quite happy practicing this form of loving. Sure, there are obstacles, especially when those around us keep telling us how wrong we are to live our lives this way.
Once again, not knowing what I was referring to (or rather pretending not to know) makes you completely unqualified in giving your opinion on the topic. You either don't understand or feel contemptuous of such a lifestyle... most likely both. You'd never see me posting on a thread about finance or cooking, because I'd come across as a serious idiot. You don't seem to exercise that same kind of restraint.
Finally, it's not about her or Brash picking up someone. It's about the dynamic that exists in this particular triad, how this new person is affecting the primary couple's dynamic, and how Blissful feels her boundaries are not respected. Being poly myself, I saw that right away. I've dealt with similar situations, from both sides. Someone coming from a monogamous POV is not likely to see that.