You can be a fan of multiple teams - I am, or have been, in every sport - but you have to have a clear #1. For example, I have a ton of college basketball teams that I love, but I never, ever, ever root against Louisville. That's my team. Once they are out in the tournament, I still have rooting interest in DePaul (remember, this is a theoretical situation), Georgia Tech, Temple, Duke, Oklahoma, Michigan, Memphis State or whatever it is called now, Nova, etc. In hockey, it used to be MUCH better to root for the Wings when Stevie Y was playing, but they were and are my clear #2.
But in baseball, I have never been able to decide between the Tigers and Red Sox, because I feel exactly the same about them, so I just consider myself a casual. Same with college football - Florida State, Notre Dame and Michigan are the same to me, so I'm just a casual, terrible college football fan.