You don't necessarily have to put all your characters in relationships, though. So there could be straight supporting characters who are single, or whose partners are offscreen most of the time. Depending on their role in the story it might not matter to the plot if they're gay or straight, so you could just not get into it. (Does this even make sense?)
Re: Finding it hard to identify with people/characters of a different sexual orientation?
You don't necessarily have to put all your characters in relationships, though. So there could be straight supporting characters who are single, or whose partners are offscreen most of the time. Depending on their role in the story it might not matter to the plot if they're gay or straight, so you could just not get into it. (Does this even make sense?)