Not necessarily, but possibly. I have always been aroused to full erection merely from oral or digital stimulation on the outside of my anus. Even a slight penetration of my anus by either tongue or finger will result in a rock-hard erection.
This is a hot video. I love how his penis gets erect and stays that way as she eats his asshole. I'd love to kneel over her and take his hard cock into my mouth and deep throat that sexy head. His mushroom head would feel awesome in the back of my throat and would probably hit my prostate good too.