I'm not sure if your boyfriend has ever had kids of his own but that may be a factor.
The big thing that I see could be holding him back.....discretion. I wouldn't want to spend my life trying to hide my relationship..I did that before when I had an affair. It is draining and always left a sense of doing the wrong thing in my mind. He needs to find his own sense of "normalcy" in this. How can anyone truly commit to a relationship they have to hide?
Me and Redpepper have friends who see us as a couple even though they know and like her husband. We have our own little world so to speak.
If you want him forever you need to be prepared to take some heat. That is a high price to pay but might be a way to give him a sense that he can be fulfilled with you.
Of course, he may just be so mono that he ultimately needs love and commitment returned in the same way he gives it. If that is the case then he will in fact replace you if he finds the right person...maybe not the perfect person but someone who he can build forever with (as long as that lasts).
Find out what he is missing. See if it can be found within your relationship dynamic. Be honest.
Playing the Game of Life with Monopoly rules.
Monogamy might just be in my genes
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