Personally, I dunno if Sulu is straight or bi, it never came up, but his friendship with Rand was camp and cheeky. It would have been nice to see more of that in TOS.
It came up obliquely in "Mudd's Women" when he says "How
I noticed." And it literally came up in ST:TMP when Ilia enters the bridge, to Sulu's distress.
I think the only aired events that call the character's interests into question are:
- The way he cradled Chekov's head in "The Tholian Web," which most straight guys would not do.
- The scene in "The Way to Eden" when he zinged a hot girl with "How do you know what I want?" Most straight guys feel like it goes without saying, she knows what we want.