LGBT(and QIA, etc) is not a genre. It is an acronym; and if you see it in a book search category, most likely key characters self-identify or are identified by others as lesbian, gay, bisexual, or transgender. If you are looking for those characters on the actual or virtual shelf, the acronym is a useful tag. It will not, however, tell you type of story and story conventions you will find those characters involved in.
A literary genre is a “type or category marked by certain shared features or conventions.” Really broadly, there are fiction, non-fiction, and poetry. When someone says, “genre fiction,” we drop to the next level of main categories, such as mystery, romance, horror, comedy, speculative (science fiction and fantasy), etc. Under each of these umbrellas are sub-genres, such as mystery: cozy, police procedural, private eye, historical, amateur sleuth, caper, romantic suspense, medical/legal, etc. A reader might, then, love a British village story a la Agatha Christie, but find a tales of lawyers in peril a la John Grisham quite tedious – or vice versa. If you know you enjoy certain elements in a story, understanding sub-genres is handy.
LGBT is not a genre is the same way that Christian, African-American, Young Adult, etc are not genres. They are better described as target audiences; and overlap with genre only in that you know that main characters in these stories will share key characteristics with the target audience. In the case of Young Adult, readers know that these characters may be found in a mystery, a thriller, science fiction, or just about any other genre.
When book marketers say, “LGBT fiction” however, they almost always mean a coming of age story, or a romance. Since I’m not keen on either of those genres, the tag steers me away from them. What I want, as a reader, is to find engaging stories in genres I enjoy, such as mystery and action thriller, and steampunk, with great characters. And some of the time, I’d like those characters to be complex individuals who don’t identify as straight.
If Girl with a Dragon Tattoo had been marketed as ‘LGBT fiction,’ would it have become a best-seller? If A Grave Talent, the first of the (police detective) Kate Martinelli series, had been categorized as ‘LGBT fiction’ rather than as a taut mystery with a refreshingly well-rounded lesbian protagonist, would it have won an Edgar Award for Best First Mystery? I think the answer is no, in the same way that Tony Hillerman’s series featuring Navajo police officers Joe Leaphorn and Jim Chee, or Walter Mosley’s Easy Rawlins series would not have taken off if they had been marketed as appealing primarily to Native Americans or African-Americans, respectively.
When the next really gripping mystery featuring a courageous, honest lawyer up against a cut-throat corporate defense team reaches the Amazon or Goodreads database, how will I find it? Will it be tagged LGBT because my kick-ass protagonist is a trans man or woman? If it shows up only under mystery, how will I find it in the sea of mysteries with cis-gendered protagonists?
If I were an avid romance reader, these kinds of multi-tag layers are finally available from key book marketers and reviewers. Since I read and write in other genres, the inability to sift meaningfully through the broad market of my preferred genres still frustrates me.