I always found the concept of being a "real fan" of more than a single club pretty odd and pretty American. No offense. Either you fully identify with and love something or someone, or you don't. Usually that feeling is pretty exclusive.
Ofc, there are some clubs I tend to enjoy watching, because they play attractively or have players I like for their skills. But that would never make me say "I'm a fan".
Well, I think a lot of it is definitely a cultural thing. You being from Germany, a big footballing country, you probably became a Bayern supporter by growing up around it, right? And so you could never dream of really being a fan of another club. For those of us who are from countries where football isn't a popular sport (not necessarily only the US), we are probably going to view things differently. I like watching several different leagues and I have a team I support in each.
Anyway, I don't have a problem with people who disagree. My issue lies with those who
don't take into account cultural differences and attack those who have a different feeling about football or club support. The only thing I personally find a little odd are those who claim to support two clubs who are supposed to be rivals - I've met people who claim to like both Barca and Madrid, for example - at that point I think a lot of the passion behind club support is lost. I'd never support two clubs from the same league either.