No you're not all doomed to life-long misery with your exclusive, long-term partner. I just don't think it's a wise path to choose if you want to be happy for the rest of your life. But you're not stating that from a position of experience and authority, that's my point. I personally don't think polyamory is a wise path to choose if you want to be happy for the rest of your life either; but I accept that it's entirely possible for you to find happiness in that as what you want and need in life may differ from my wants and needs. I also have never tried having multiple sexual partners in an open relationship. I think it could also be argued that you're not necessarily qualified to argue in favor of monogamy either though. You've been in a relationship for 4.5 years, but if you're assuming you'll be together for the rest of your lives, you're still in the relationship's infancy. There's still lots left for you to experience... moving in together, getting married, having kids, etc. And vice versa, you're still in the infancy of your polyamorous lifestyle. I can do this all day :P