Well, for starters, Sulu shows quite an interest in Uhura in both "The Naked Time" and "Mirror, Mirror." (And yeah, it was the Mirror-Sulu in that episode, but no one commented or expressed any surprise that Mirror-Sulu was interested in a woman.)
Sulu is also affected by the women in "Mudd's Women," like every other man on the ship:
He also shows quite a bit of attraction to Lt. Ilia in the longer edition of TMP, to the point of hitting the wrong button on his console.
His daughter is seen at the beginning of
Generations, and we're told that Sulu had a family. (This wouldn't be the same daughter we see in
Star Trek Beyond, as the ages don't match up.) Presumably Demora was intended to be his biological daughter from a heterosexual marriage.
Sulu also had heterosexual romances in the novels
The Entropy Effect and
The Captain's Daughter, and in several Star Trek comics, including an annual co-written by George Takei himself.
So yeah, there is evidence that Sulu was in all probability heterosexual.
As Takei said in
many interviews at the time, he didn't like a previously-established character being "revealed" as gay. He thought it went against the original intentions of Gene Roddenberry, who created the character of Sulu, and he believed that it carried the potential connotation that Sulu had been closeted for years. There's also the unfortunate implication of "Oh, the actor is/was gay, so therefore his character must be too." It's rather disrespectful to Mr. Takei as an actor to imply that he can only play characters just like himself.
Takei was all for more gay characters in
Star Trek. He just thought that it should have been a brand new character instead of Sulu.
