He's not dense. He's actively denying and rejecting it. He doesn't want to randomly get into a relationship with the junior when he doesn't know what to do with Sayu. Since he allowed her to live in his apartment, fed and clothed her, he assumed responsibility for her. It would be worse to suddenly kick her out than not take her in in the first place. If he started dating some woman out there, how could he possibly still keep Sayu around? So, the only solution is not to date anyone haphazardly for the time being.