There is a big difference between being ashamed/embarrassed of someone and being ashamed/embarrassed about a situation. From what I can tell it is not you that he had the problem with...it was the relationship dynamic. It's easy for me to say don't take it personally" (because I am often very Vulcan in how I see things) but please keep my comment in mind. Use it as a mantra "It was not me, it was the situation". Remove your self doubt from the real issue...you're boyfriend wants a more conventional arrangement.
Playing the Game of Life with Monopoly rules.
Monogamy might just be in my genes
Poly Events All Over