I went through a similar situation as you, Villo. It isn't very comforting when things start off weird. My girlfriend and I were monogamous for five years, and then started exploring just two years ago. At first, I was against the idea, but after research and thinking, I started to accept it more and more. I think (for me, so I can't speak for anyone else here) setting a few ground rules is a good way to start, just so you can both meet in the middle. To give you one example, my girlfriend and I agreed that it shouldn't be an excuse to sleep with anyone they want. If she is attracted to someone, and something happens, have the respect to say something about it BEFORE anything happens.
There are three main things that MUST happen in a poly relationship. Trust, respect, and communication. These three things are crucial to keeping a polyamorous relationship strong. Even if one party has the tiniest doubt, it should be mentioned to the other parties involved. I personally don't like the "If you don't like it, tough cookies. I'm doing it anyways." That's a big stress in and of itself. I personally believe that once you vent, and lay everything out on the table, a lot of that stress will be lifted off your shoulders, and you put the ball in THEIR court. "What's meant to be, is meant to be."
Keep your chin up, Villo! PM me at anytime if you need to vent.