I would say no, do not let yourself think that you caused this. I'm not sure anyone here was in a poly relationship, but it's easy to deceive ourselves into thinking if we did more of this or less of that, then we could have avoided what happened. That's not reality. Orientation is inside a person, I don't think anyone 'makes' a person turn gay, lesbian, poly, or anything else we often see here. If someone has a need for another partner, of whatever sex, it was bound to come out unless they were really disciplined and committed themselves to monogamy. If you are wondering if you set the right boundary, I'd say you have the right to want monogamy. You have to be true to yourself. Self-sacrifice is not noble if it crushes your soul. If your spouse is content with the boundaries you have, it sounds like she is committed. If you are both working to maintain this relationship, I think you would be OK. Don't burn your energy pondering the road not taken. Focus on the road ahead. Be well.