A gay man who is also a whore. Mainly used to insult another male who is not acting "manly". This can be insulting to either gay or straight men, or both.
One step beyond fanboy or fangirl, a fanwhore is someone who is so insanely obsessed with something they treat it practically like a religion and will physically harm others who disagree with it.
Twilight fanwhore: "OMG TWILIHTGT IS LIEK THE BESTEST BOOK EVAR1!11! STEPHANY MIYER IZ A GENIOIS!111"
Sane person: "Twilight sucks ass, so shut up about it your retarded fanwhore"
Twilight fanwhore: "OMFG HOW FUKIN DAR U INSULT TEH GRAT GODESS STEFANIE MIYER I WIL FUKKIN KIL U!!11!"
*Twilight fanwhore proceeds to kick sane person in the balls*