In my opinion, that's just cheating. If your partner isn't open about their other partners from the beginning, and doesn't give you a chance to decide if you want to carry on or not, then they've just cheated on you behind your back. And you're well in your rights to be upset and angry at that.
Oh I consider it cheating all right, but the thing is, she already had multiple relationships at the time, and I wasn't even her "main". In fact, the one who was her main gf admitted to being the one pushing it on her. Long story short, it all ended in a big fight.
But my opinion still stands. It's just how I am, I need to be in a couple and not a quad or any other group relationship. I get attached, like intimacy and being special to someone, and have bad jealousy issues. I understand poly is some people's thing, but I know it's not mine.