Is the master cheif gay

Doubtful, if you read the Halo: The Fall of Reach it mentions that all the Spartans underwent surgeries and one in particular was on their sex organs. I take this to assume that ONI found a way to turn off their sex drive so it wouldn't distract them during war. Also they enlarged his penis size to 15 inches long because it increases self confidence and to make him have a "super weapon."