Beat me to it. After thinking about it, I was left thinking the same.
I've been following City since '69 and can say that I don't think we've really got a proper footballing rival. And certainly not in the sense of say Newcastle / Sunderland, Norwich / Ipswich, Pompy / Southampton, the obvious big city rivalries and Leeds / Bradford. As others have said, there's a scale of dislike of certain clubs, but that doesn't make for rivalries.
It's wrong and a real shame that there is, albeit as a generality, this Hull RL / City 'rivalry'. Maybe if there was only 1 RL club in the city it might be different.
Of course, it should be one of FC / KR + City ..... and vice versa, even if it's only a passive +. The Hull RL / City 'rivalry' simply smacks of jealousy, small mindedness, scarcity mentality.