Okay: Having a favorite band member and it being the singer.
Not okay: Disrespecting the other band members.

Okay: Only knowing one song by the band.
Not okay: Claiming you're their "biggest fan"

Okay: Expressing that you think a band member is attractive
Not okay: OMGZZZZ LEMME HAVE YOUR BABIEZZZ FUCCKKK MEE

Okay: Not liking a band members love interest.
Not okay: Sending him/her hate messages.

Okay: Not liking a band
Not okay: Harrassing the people who do like the band for no reason.

Last thing: If someone's favorite song happens to be one of the more popular songs by the band, that does not make them any less of a fan. King For a Day, Weightless, and Second and Sebring (for example) are all incredible songs. If they weren't then they wouldn't be singles. Don't say someone isn't a "real fan" just because they like one of those songs the best