This is diversity for diversity's sake (as I've said before):
"The makers of the rebooted Star Trek movies decided to make an established character (Sulu) gay, completely out of the blue, in the 3rd movie of their franchise. They asked the actor, George Takei, who played Sulu in the original series and movies for his blessing, thinking they would get it easily since he is openly gay in real life. He told them very clearly, that he portrayed the character as a heterosexual male throughout his entire career, because that was the character that Gene Roddenberry created. Takei said that he was all for an openly gay character in Star Trek, but that they should create their own character rather than paste the sexuality onto an existing one. The new Star Trek filmmakers, who claimed they wanted Takei's blessing in the first place, turned around and made their version of Sulu gay anyway. That was a complete slap in the face to George Takei, basically telling him, "You're gay, so Sulu must also be gay too." It's also a complete insult to Takei's acting abilities, saying that he couldn't possibly have been portraying a character all those years that wasn't a complete match of his actual sexuality. The rebooted Star Trek doesn't even take place in an alternate universe where this sudden change would make sense. It's only supposed to be an alternate timeline. Sulu was an adult male already when Spock and Nero went back in time and altered the timeline. How did Nero going back in time suddenly make Sulu gay? If they really were doing it as an homage to George Takei as they claimed they were, they would have respected his wishes when he said he was against it. They could have easily created a new character as Takei suggested, but instead they forced a sexuality change on a character who was portrayed as a heterosexual on both TV and film for three decades. That's not being respectful to the source material, the actor who defined and brought life to the character, or even to good storytelling, since the sudden change is completely unexplainable. This example is the very definition of forced diversity."
Was he ever explicitly heterosexual? Or did we just assume that he was since that's what is considered the default?
I'll take George Takei's word for it that he was playing a heterosexual character since he played the role for three decades. I won't belittle his acting ability by saying someone other than himself or Gene Roddenberry knows better about anything regarding the character of Hikaru Sulu than them.
George Takei: ""Unfortunately, it’s a twisting of Gene’s creation, to which he put in so much thought. I think it’s really unfortunate. I told him, 'Be imaginative and create a character who has a history of being gay, rather than Sulu, who had been straight all this time, suddenly being revealed as being closeted.' I said, 'This movie is going to be coming out on the 50th anniversary of Star Trek, the 50th anniversary of paying tribute to Gene Roddenberry, the man whose vision it was carried us through half a century. Honor him and create a new character.' I urged them."