Well, I hope I don’t sound flippant, but this forum is a very good place to find out about how to write LGBTQ+ characters and relationships. And, for the most part, they’re going to be pretty much the same as straight and cisgender characters. In fact, letting the player romance whoever they want to is actually easier to code than coding the game to ensure that only straight relationships are allowed. 
They/them pronouns for nonbinary characters is a little harder due to using plural verbs, but you won’t generally be referring to the MC using the third person, so it’s still not that big an issue. 
If you want to find out more, you can read or ask questions in some of the following threads (among others), or start your own to ask:
To be clear, I’m not saying you have to include LGBTQ+ characters and relationships, but isn’t hard to write or code, and you’ll get a lot more readers if you do. Good luck with your game! 