Is the Emcee gay?

One view you could take is he was just a Pied Piper. The MC who was only concerned with doing his job. Androgynous, all the naughtiness on the stage when he joined in like the two ladies routine, just his job to keep things flowing and sometimes doing his own shtick.

Thats one take, but it hits me that beneath all that makeup and leering, there was a quiet, reserved soul who may have been a real nice fellow, who when his shift ended, may have gone home to a hot bowl of soup and reading the paper. You never saw him fraternizing like Sally did or some of the performers.

Lot of possibilities.

I think he was symbolising 1920s decadence.
The cabaret would not have been considered wholesome and would presumably have been closed down when the Nazis came to power. In the last scene of the film, Stormtroopers are in the audience, reflected in the metal of a musical instrument. At least as a veneer, the Nazis were socially conservative, often expressed hostility to big city values and blamed Weimar decadence on the Jews. The MC might indeed have ended up in a concentration camp if they thought the effeminacy was more than just an act, and even as a stage act it would have been forbidden. The stage show where they parody militarism would not have gone down well either, and even before the Nazis achieved power Nazis in audiences were known to disrupt stage shows or films that ridiculed the armed forces.

"Chicken soup - with a *beep* straw."

In the original stage production he's mostly an amoral slease, at least on stage.
That's what I get from Joel Gray's performance. He was in both the original stage
version and the film. In later stage versions he is more androgynous suggesting bi-sexuality.
Some directors/actors have speculated on his backstory and fate. He makes fun of
Nazis and Jews, maybe he's secretly Jewish(thus the yellow Star of David) and the
androgyny suggests he's bi-sexual/gay(the Nazis wouldn't make the distinction)
(thus the pink triangle).