I think the daughter factor, even if not a direct indicator,
Homosexual couples can have children even in this day and age. Again though a 1994 assumption of what a traditional family structure is might well be different from that of the 23rd and 24th century.
If you looked at it the other way and showed someone from before the American Civil War an episode of TNG they might well assume that La Forge was a slave forced to shovel coal in to the Enterprise's engines rather than a senior officer running a department with a staff of hundreds.
If you are going to make assumptions about a fictional character you need to consider the world and the time that they live in.
and the fact that the writers are doing this for the wrong reasons, are enough for people to scoff at this.
I respectfully disagree that that is a 'fact', it's the opinion of some people.
If it had previously been established that Sulu was 100% heterosexual and could not, for whatever reason, marry a man then I would understand people objecting about changing an established character, I might not agree as the character is 50 years old so I wouldn't expect them to stay exactly the same, but I would understand the objections on those grounds.
The writer's are not changing Sulu (unless anyone can provide proof that he couldn't marry a man), they are just revealing that he is married to a male. That doesn't even necessarily make him homosexual, he could be bi-sexual for example, since Star Trek Beyond hasn't yet been released we haven't seen how Sulu is labelled if he even is at all, all we know is that Pegg has written him as having a male husband.
This is one of the things I love about the current generation of Star Trek books, they are organically very progressive and inclusive, characters aren't labelled they just are, there is no straight or gay there is just people that are attracted or love certain people. My personal belief is that is how the world should be.