Addiction is very difficult to define. I don't question there are people for whom sexual behavior is a problem, but the extent to which this overlaps with drug addiction is not well established. This extends across all behavioral addictions quite frankly. Though oddly enough, most people don't question whether "gambling addiction" is a thing but do question many others (internet addiction, sex addiction). Fundamentally, there isn't any obvious reason to separate these. In some ways, the existence of "sex addiction" would actually be more logical (UR with clear physiological effects). I think the main reason gambling disorders have more street cred are just because more research has been done on them (which historically is likely due to the prevalence of the disorder, the nature of the problems it causes and the stigma surrounding sexual behavior...none of which have anything to do with nature itself).
That said, all psychopathology is messy. Those of you doing addiction work, how many "pure" cases do you see? Extraordinarily rare in my practice - I pretty much only saw them in forensic settings where it was arguable whether the person would have even met for a use disorder if not for societal consequences stemming from drug use. Evidence is even mixed with regards to whether drug addiction is fundamentally different in a manner driven by drugs themselves (see Carl Hart's writing, rat park studies however flawed they were, etc.).
My personal belief (and research program) is guided by the belief that all these "types of issues" are a constellation of impulse control, emotion dysregulation, cognitive deficits and other basic processes overlaid on physiological effects (from drugs or otherwise) and modulated by micro (family peer, etc.) and macro (legal, societal) environmental features. When you take a view like this, what is and isn't a disorder becomes arbitrary silliness we settle for because there are societal benefits to oversimplifying things in our current environment. We're a long ways from "carving nature at its joints." Doubt we'll get there in my lifetime, though hoping to move us closer.