It's all about wearing people down at every turn by opposing whatever choice is made no matter what it is or how it's done.
If the role is large, oppose it as pushing an agenda.
If the role is small, oppose it as not doing enough and say they should save it for a bigger character.
If an established character is announced to be gay, oppose it for violating canon or someone's vision.
If a new character is announced to be gay, say that it's tokenism.
Or oppose it because it's in the Abramsverse (sorry, Kelvinverse) and everything about that sucks.
Or oppose it because the makers are hacks.
Or oppose it because a lot of Trekkies are unfortunately allergic to change of any kind.
The goal is to just keep the opposition going until everyone is fed up with talking about it and you've soured the whole idea, so maybe next time producers or writers decide that they don't want to deal with all the drama and just don't bother with a gay character (or all-female Ghostbusters, or black Johnny Storm, or a female Starbuck, or a Latino Spider-Man).
tl;dr - It's all about gay people even existing.