I figured he was keeping her gay for the simple consistently. I don't think they really need to use the show to "spread LGBT awareness" and to be honest, The Last of Us is not really a great story to do that with. It's a society where along with these types of social and political stigma that come with it with regards to sexuality, race and so on has effectively broken down. Most people in this would happily kill you for a fresh pair of shoes, there are bigger, or at least more direct problems! That's why the game doesn't put hardly any focus on in it, hell the fact that Ellie is gay wasn't even addressed at all until the Left Behind DLC came out.
Like I said earlier The Last of Us is a story about the relationship between Joel and Ellie and how it evolves as they journey together. We see Joel learn to love and care again, something he has been struggling to do since the loss of his daughter. Likewise we see Ellie, a young girl who is fiercely independent and stubborn learn to trust and open up to him. That's what the Last of Us is about, it doesn't need a ton of focus on Ellie's sexuality, at least not in this story.
Now the sequel (judging from the trailers anyway) does seem to be putting a bit more focus on it but we don't know exactly how that's going play out yet.
hope this helps:
https://www.netflix.com/CancelPlan
Now if you'll excuse me, I'm going to listen to some music: