Well, I had imagined something like "taking infidelity into account, monogamy's conception of exclusiveness doesn't seem to work perfectly either". I guess that another person might have just been irked by the prospect of not being considered, by herself, "(good) enough" for the person who made a move on her, (in retrospect, you seem to find nice that, given new information, he would have agreed to a monogamous relationship, meaning that he considers you sufficient to satisfy his romantic expectations). Focusing on that would have been counterproductive, in this instance, since it would have probably made it impossible for him to date you at the time.