Look, the point is that “realistic” might not be the right word, a better term would be “believable”.
You might be familiar with the idea of “Suspension of disbelief”, right now I don’t want to discuss the whole concept, but the basic idea is that a storyteller pretends that the story is real and the audience pretends that they’re believing it. It is considered a failure on part of the storyteller when some element disrupts this and the reader needs to remind “well, this is just a work of fiction”. And this has nothing to do with realism, any story can fail to be believable whether it takes place in real life or in a fantasy land.
You might also have noticed that this can happen the other way around, a particular scenario can be considered unbelievable because “It’s impossible that no character in the entire place happens to be gay” or more often “It’s impossible that all main characters are always male”.
But to go back to your original issue, are there works where the inclusion of LGBTQ characters might disrupt the suspension of disbelief? Well, I’m inclined to say no. I think that part of the reason why some people may take an issue with this is that we are not very used to see LGBTQ characters in most scenarios, as you said:
However there might be some truth that in some scenarios, LGBTQ people is expected to be trated diferently, this sometimes entails the portrayal of discrimination. Some CoG games have adressed this, there are some differences in having a gay romance in games like Choice of Broadsides, Choice of the Vampire or A Study in Steampunk (I think, I’m not entirely sure right now). So I think, it can be done, it just needs to be done carefully.
Personally, I also want to consider some opinons like this one:
and this one: