"Hard science fiction generally isn't fascist, but one can make a
plausible case that Star Trek being fascist.
A) They have no politics. It's a military dictatorship.
B) They have no money. It's a command economy.
C) All conflict is racial. Humans v. Klingons v. Romulans etc.
D) The races have intrinsic cultural personalities which make them less
attractive than the humans. Attractive members of alternate races try to
become more human: Spock and Worf trying to get a sense of humor. Data
trying to get emotions.
E) Something terrible happened to Asians, Africans and Latins, because
90% of all humans are English-speaking whites.
F) Cognitive enhancement and life extension technologies are outlawed,
or at least all R&D towards those goals have been stopped.
G) Everyone apparently has some kind of mind-block against realizing
that the transporter beam could make copies of all the crew and keep
them young and immortal.
H) The Prime Directive maintains human (and allied) supremacy over the
hapless lesser peoples who are denied political and technological
progress in order supposedly to respect their cultural "difference."
However Kirk does go back to 1939 and stands aside to let his pacifist
girlfriend to get run down by a car instead of allowing her to convince
Roosevelt to stay out of WWII. So at least Star Trek wasn't Nazi."
See Brian Wang's response here.