Okay, I know I'm probably gonna piss some people off by saying this, but those people probably don't stop by here anyway, so here goes... If your character's sexuality has nothing to do with the story, then you don't need to mention it. Not in the story. Not outside the story. Not after the story's over. No need to do a big reveal there, JK - it wasn't pertinent to the story. Ever.
But let's say it is pertinent to your story in some way. If your character is gay or bi, show them having romantic feelings toward a member of the same sex. (Although, I'd only really think it was pertinent if you were writing romance - with suspense after or paranormal before or whatever). Show a dude going out with his boyfriend. Show a gal checking out another gal. Make it smooth. No need to beat your readers over the heads with it. If you don't feel like you've gotten the point across without being explicit, work on your writing.
Unless, of course, you FEEL THE NEED to beat people over the heads with it. Then, perhaps, you need to think about why you feel that way and whether you're writing fiction or proselytizing. Personally, I prefer the fiction. If I wanted the other thing, I can find it in plenty of other places. Leave it out of the fiction.
Yeah, I DNF'd a book yesterday where the author came out with a good phrase about the female MC - she found a woman beautiful but the gal wasn't her type, but they'd already said she was living with a man, so I got the point there. Then a couple pages later, the author actually came out and said the character was 'bi'. Why? I didn't miss it before, so why did it need to be explicit? Because things like that have to be explicit in certain realms or you don't get credit? :shrug:
That's poor writing, by the way. If you can show it another way, you don't need to tell it. Show, don't tell. How many times have we writers heard or read that? I couldn't begin to count over the past 18 years how many times I've heard it. So many times, it's embedded in my head now.
Personally, I don't care about a character's personal tastes in bed. Unless I'm reading a romance and then I prefer the sexy bits align with my own preferences. Same sex romances don't interest me, but more power to ya. If it's not a romance... whatever floats your boats. I used to enjoy an urban fantasy series* where the sidekick was gay. She wove it in rather than tell the reader about it and it was just part of his personality. Gnarly. Write on, McDuff. Actually, thinking about it now, I've read a lot of books where a character was gay and they were written as part of their personality. It wasn't jammed in there like someone was checking a special box. That's the way it ought to be done.
In short, and I know I've said this before, the story is the thing. Write the story, populate it with the people it requires, and write it to the best of your ability. Show don't tell. Etc. etc. etc.
* I say 'used to' because she stopped writing it. Then I saw on social media where she'd gone full on ACTIVIST, so I didn't bother reading anything else by her**. I enjoyed the stories but I don't enjoy indoctrination. K thx bye.
** This has, unfortunately, happened a couple of times. It makes me sad.