Being gay doesn't change his character. Alex being gay doesn't change her character. That is what I am confused about with your argument. LeFou is a character who functions to fawn over Gaston. He admires him, he "loves" him. Him being gay does not change that. If anything it makes the devotion a bit more clear.
Alex's character on Supergirl was not changed with her being gay. Her romantic life was a non-issue and she did not react to men in a romantic manner. As others mentioned, it was kind of hinted at in the first season.
So, would you be ok if a character was gay in the original story, and then many years later they remake/adapt the story and make the character straight?
What if they adapted Alex Danvers into the Supergirl comics ( it already has Eliza and Jeremiah Danvers and the DEO ), but made Alex straight instead of gay as she's in the show?
They don't really change anything about the character other than her being straight instead of gay. Instead of being Maggie's lover, they are now just best friends.
Would you say she's still the same character?
Largely, she would still be the same character. But you can't say that the dynamic between her and Maggie wouldn't change by making Alex straight instead of Maggie's gay lover.
It does change the character. Maybe not the character itself but the character's relationships with other characters.
And LeFou fawned, admired, and "loved" Gaston in the original film without having to be gay. I never got the impression he was gay, but if someone did, that means it was subtle and implied and left open to interpretation. Which is how they could have left it in this film.
Making LeFou gay actually does change the character, I'd argue, because now it adds a level of sexual/physical/romantic attraction. LeFou now doesn't just admire Gaston because he thinks Gaston is the best, most awesome, most coolest guy in town. He now admires Gaston because he's actually in love with and attracted to Gaston.
So, imo, that's a change. It's a change that fits the character, but it's still a change. LeFou himself maybe largely the same character, but now his relationship with Gaston takes on a new dynamic that wasn't present before.