You are important and you matter. Your feelings matter. Your voice matters. Your story matters. Your life matters. Always.
First of all, you have to be completely honest with yourself: What will you accept from a partner in an exclusive relationship? Is contact with an X ok? If so, what kind of contact? (Facebook, emails, telephone calls, meals, etc.) If any of this is OK for you, do you expect to be notified of the contact? And on and on...
Then, when you have the exclusivity talk, IMO you should lay out your terms (and you should encourage your partner to do so.) That way there can be no misunderstandings or mixups. If you can't come to a meeting of the minds at this exclusivity point, you aren't meant for each other.
Let's face it. At this point in our lives, we all have pasts and baggage, for lack of a better term. Characters are going to come out of our pasts, and our SOs will have to deal with them at some point.
Me: FBS (no longer betrayed nor a spouse)-62
D-day: 2007 (two years before finding SI)
S: 6/2010; D: 3/2011