Folks assume that Sulu is heterosexual not because there's been significant evidence presented to suggest that he is, but because there's been no significant evidence presented suggesting that he isn't, it's an assumption reflective of the heteronormative nature of society.
Well put.
Also, Sulu's sexual orientation has never been an essential part of his persona. It would be weird to make Kirk gay, because he has this forty-year-old reputation as a great ladie's man. And Spock has always been torn between his Vulcan logic and his human feelings for various women on the old show. (He had at least one doomed "romance" every season.)
But characters like Scotty or Sulu or Chekov? They were never defined by their sexuality, but by their job descriptions and schticks. Scotty was the the jovial Scotsman who kept the engines running; Sulu was the cool navigator who once displayed an interest in fencing; Chekov was the green young ensign with the funny accent . . . .
Nobody thinks of Sulu as "the Asian guy who likes chicks."
(And, no, the Mirror Universe doesn't count. Those people will sleep with anything that moves . . . )