Who cares? Why does it matter? The answer has everything to do with respect. First, your professor worked very hard for their degree, and in doing so, built up a wealth of expertise. Addressing your professor by their appropriate title shows acknowledgement of that effort and expertise. Second, for women, people of color, queer or LBGTQIA folks, and other marginalized people in the academy, their fight has often been an uphill battle in institutions that are not built for them or for people like them. These faculty members often face discrimination and bias that follows them throughout their careers and works against their promotion and tenure (see literature on bias in student evals, for instance, here). Referring to these faculty members appropriately acknowledges their place in the academy and their hard-won position of (again) expertise. And if you're not sure about your professors degree, then "professor last-name," is always a good choice.