For my part I prefer it when we're not concerned with faithfulness as a serious issue. I acknowledge my biases, but it is for that reason that I can confidently state it is neither about best girl preferences nor my haremfaggotry when I say I cannot bring myself to care. Crab and Araragi's relationship is simply no longer compelling - not as a plot point, nor even as an aspect of the setting. Disregarding any specific opposition to it, their chemistry has, with only a few exceptions, grown stale and any drama that drives their relationship to the forefront feels frustrating and distracting, rather than serving to generate interest. As I said, I have my own biases, but in this case that she is his girlfriend isn't the issue - rather, the issue is that we are consistently reminded of it in unappealing ways.