Takei took the minor role because it made sense for Sulu as a character, as well as personally the representation of a Japanese (and then closeted gay man) as the captain of a starship.
Takei has suggested that while he is gay, Sulu is not. He objected to Sulu being gay in the Abramsverse and said they should have made a new character gay instead of shoehorning it into his character just because he’s a gay actor.
Which makes sense. Sulu could, at best, be bisexual. Not because there was no gay representation in TOS, but because he was shown being interested in women. He’s attracted to the titular women in Mudd’s Women, he “takes over” with one of the showgirls McCoy has on his arm in Shore Leave, and he’s arguably sexually interested in Uhura, albeit while intoxicated, in The Naked Time. As Takei himself knows.
I’m not gay, but if I was and was known for playing a prominent character in a popular show and then they remade the show as a movie, but made my character gay because I was gay… I’d be kind of insulted. It would basically suggest that every character I played was gay unless clearly indicated otherwise, which is not what I would want people to think of my acting abilities.