AntiPoly's completely unhelpful and untrue post aside, yes, you can still say you're poly though you're currently in a mono relationship. My husband MC is mono, but is in a poly relationship as an arm of our V. That doesn't make him poly. He doesn't have any other partners, nor any desire for them (if he wanted them he could have them!), so he's definitely mono. Your personal preferences are separate from your relationship choices.
Pan Female, Hinge in a V between my mono (straight) husband, Monochrome and my poly (pan) partner, ThatGuyInBlack