Definitely not a must have. I’d rather have a story that doesn’t mention sexuality then a story that forcibly shoves in LGBT characters. That being said, in a modern-day story, not everyone should be cishet 24/7 if it doesn’t matter to the story.
Back to not shoving in LGBT. Don’t put it in if it doesn’t make sense. The story I’m writing right now takes place in the Civil War, which most definitely was NOT an LGBT-friendly time, so I’m not gonna put in any (openly) LGBT characters.