A character transcends their actor or their creator. They are what they story needs them to be, provided it fits within what has been established. Just because Sulu hasn’t been shown as gay before doesn’t mean it can’t fit within what has been established. And, while I know when Takei says Gene’s original vision he’s talking about the very specific fact of Sulu’s orientation, I do believe this change is very much in keeping with Gene’s larger vision (unless his vision for an all inclusive Federation somehow excluded gay people?).
Let us not forget that this is not an alternate universe. This is supposedly just a divergent timeline. Sulu has always been heterosexual, he even has a biological daughter.
If anyone knows what Gene’s vision for the character was it would be the man who played the character for over three decades. To reveal in this film that Sulu is gay would mean that he was closeted throughout the entire original series as Takei points out himself. That doesn’t really sound like something that would happen in Gene’s vision of the 23rd century does it? Again, something that Takei pointed out.
The character of Sulu is very close to Takei’s heart. The fan film World Enough and Time (Highly recommended) stars Takei and solidly cements Sulu as heterosexual. While not canonical, Takei is very proud of this production and more than likely considers it part of his personal canon for the character. For the filmmakers to approach Takei for his input/blessing on something that they state is meant to honor him and then completely disregard his wishes is disrespectful.