Sometimes one's irrational hatred blinds one to & of the more then underdog-ish state of one's team, but that is usually stupidity or some trauma & not homerism.
On the side of stupidity, some one once said, "it ain't braggin if you can back it up." If you team is competing against a better team & you know it, then you are just being a fan rooting for your team. If you have figured out how they can win then your hope is not without merit. But if you are just blindly saying the better team sucks then you might get the homer tag applied.
In the instance of trauma, for instance, my childhood experiences of Woody Hayes forever made me hate all things Buckeye. I think that that is just irrationality cuz I don't just root for PSU against them, I root for everyone who plays against them!
The biggest "homer" I ever heard was the late Bob Prince, the announcer for the Pittsburgh Pirates back in the 60s & 70s. Being a Mets fan growing up in Central PA made me hate the "Buccos" but that was probably due more to the fact that back then the Pirates were just plain good & less listening to Prince, (though he made it that much more unbearable).
I think a "homer" is a fan who doesn't realize that in fact other teams exist. Hence this is why I believe most Hawkeyes are homers. Perhaps their success has blinded them but thats my Reader's Digest analysis of a homer is & I would agree with Gold that any fan has a little of that in them @ times.