lgbt people exist, and its realistic to include them in your stories even if ONLY as side characters mentioned only once.
but i also wanna point out that so much writing about lgbt people is just- stereotyped?? like not all gay people are sassy and flamboyant. come on.
but ALSO try not to make EVERYONE gay like not many people need this advice but some of yall go overboard and then also suddenly everyone is dating each other too!! come onnn. i have a lot of lgbt friends (myself included) and even then were not in some weird gay love triangle/square/rectangle and living happily ever after chiiiill
on writing prejudice in this matter you have to consider a lot of nuance. different people are prejudiced in different ways. FOR EXAMPLE: my mom is prejudiced against lgbt people. does she hate them? no. is it because of religion? no! she just thinks its "weird"! she doesnt have anything against them, but still sometimes says slurs because why the fuck not!! because she can! (sorry ill reign in the vent)
point is, thats one type of homophobia/transphobia- a kind of careless ignorance that can be very hurtful, it often comes from people who arent very aware of just how bad some of the things they say are.
then theres people who try to mask their prejudice and it just gets,, cringey and weird?? like the "oh my neighbor is gay i love The Homosexuals im so woke" or the creepy ass girls who are like "omg i love gay guys i want a gay best friend so bad"
theres the middle schoolers who call u gay in the hallway. theres the actual violent people who commit hate crimes. there is such a wide range of prejudice that may appear in your writing BUT:
its very important to consider time and location. for example, if youre writing about a gay couple in 2019 LA or San Francisco, then they probably wont face much prejudice except from maybe family members or the occasional rude stranger. a book that takes place in, say, the fifties in arizona would be completely different! so yeah just stuff to consider