I think its completely unreasonable to open your marriage and then go about putting rules on it such as no previous interests or partners. You wouldn't of opened your marriage if it weren't for this guy by the sounds of it so why not call it like it is.
She cheated in you basically. That will take some time to get through. You are justified in asking for the time. Deciding to open your marriage is not going to give you the time. Asking her to let this guy go for awhile until you establish boundaries, recover from here cheating, make sure you have a solid foundation of mutual trust and respect with a whole lot of understanding of what it would mean to be "open" would be more appropriate.
It sounds like she wants more than just sex. This would mean a poly relationship. Generally "open" means open to sex with others, not emotional attachment. It sounds like you are thinking more in terms of sex with others and assume that is what she is after also. That is a whole conversation that it sounds like you haven't had let alone others about the future, relationship dynamics, what you have learned from reading and asking questions. Ya, I think that there is lots to do before even the thought of others being in either of your lives.
If you do a search and look at the tags you will find some intersting threads on "foundations" and "lessons" that might help.
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