haha flamboyant people are for us to laugh at
This is the biggest thing for me. It's the stereotype aspect of this that pisses me off more than anything else. Sexuality is not a punchline.
The biggest example of this I can recall is Game of Thrones. The knight Loras Tyrell was written by George RR Martin to be a widely praised warrior who is secretly gay and a passionate lover of the king's brother. It's rumored that he is gay but it is not his defining trait and above all else, he is loyal to his lover.
In the show, Loras is defined entirely by his sexuality and it becomes a running joke after a few seasons, in jokes that basically amount to "haha Loras likes dick". He sleeps with or flirts with most other male characters he encounters and when not being shown as a punchline for a gay joke, he's shown in a token gay hookup scene to show how "open minded" the writers are. Oberyn Martell, who is bisexual, immediately shows stereotypes as well, being portrayed as a sex-crazed player.
As someone who has struggled with their sexuality growing up, portrayals like this do plenty of harm. LGB characters when cast as stereotypes are pretty much always used as punchlines, making sexuality into a laughing matter for the amusement of straight people. Straight people defend these portrayals with comments like "well at least they're being represented at all". But that logic in the situation of a straight actor playing an LGB stereotype is pretty much the equivalent of defending white actors using blackface to portray black stereotypes for white amusement by calling it inclusive and praising the presence of a "black" character.
Straight actors could play gay characters if the writing is mature and the sexuality is not used as a laughing matter but that seems to be too much for most of Hollywood.