Hello. I believe I can be of help to you with this issue.
If you are moving toward a fully-committed and exclusive relationship with him, then there should be no other women he's been intimate with from his past, who have consistent and regular contact with him as 'best friends'. He can give up a best friend for his fiance, if it will contribute to helping cement feelings of trust. This should be no problem, and the 'friend' should understand completely and give the man her 'blessing' to disengage. She can find another best male friend. If you aren't moving toward an exclusive and permanent relationship, then you will have to figure out a way to tolerate and possibly put up with the amount of contact they have. You could suggest that for you to stay in the relationship, you need to become involved in their friendship e.g.,you do activities or any personal contacts together through face to face visits e.g., if she asks for specific physical assistance with anything.
Otherwise, you have a situation in which your boyfriend is establishing a foundation for having other women friends he secretly talks to once you are married. I doubt he would tolerate a situation after marriage in which you had old boyfriends calling, asking you to meet for lunch a few times a year, or doing other things together without him, ,and particularly without you telling him. So, what would happen if the situation was reversed?
Well, please trust your instincts on this one. He wants a semi-monogamous relationship because he is getting something emotionally satisfying from this old relationship, and I think your description---emotional affair, is quite descriptive.
You might want to show him copies of this post and see what he says.......then let me know.......he could write me back if he wished......
Incidentally, please don't forget to hit the green Accept key before you finally log off.....thanks.