Ok, this is what we know:
1) When this guy gets serious, he needs monogamy.
2) He set up the foundations for a serious relationship with you and then after some time, decided it wasn't right.
3) You have now started a more casual relationship which excludes the monogamy he requires for a serious relationship.
4) Like many people, he enjoys discussing his other relationships with his partner(s). He does this despite you saying you don't want to hear.
These four things less me to believe that either consciously or subconsciously, the threatened loss of your relationship isn't enough to persuade him to sacrifice some of how he wishes to have in his relationships for what makes you comfortable. When you are emotionally invested in a relationship, you are more likely to be willing to compromise, at least temporarily, on the little things in order to ensure your partner is at least gaining trust, security and all that jazz. Especially when the thing they want doesn't affect how you interact with other people, only how you interact with them. It goes back to what I say about obligation only becoming burdensome when you don't want to fulfill your obligations. He has an obligation to respect your boundaries, and he is clearly shirking that for what is the optimum situation for him. He just isn't that into you.