Sounds to me like you and your partner need to do a WHOLE lot more communicating before either of you progress into a Poly relationship. IMHO when you, as a couple, move toward Poly BOTH of you should feel absolutely secure and comfortable in the knowledge that, no matter what else happens, your commitment to each other will still bond you together. Period. All the niggling little questions and fears have been brought out into the open and solutions either willingly agreed to or the emotion dragon has been slayed. If BOTH of you are not moving towards Ploy with a willing heart and an eager smile with just a tinge of anticipatory fear then there needs to be more communication and happy agreement or disaster awaits the union.
That's my two one hundredths of a dollar on the topic.