So i got bored recently and wanted to figure this stupid question out, here we go:
Starting off with gender identity, after looking at the wiki for a sec the one with the most characters is "Transmasculine" at 14. This one means you were assigned female at birth, but now identify as male.
Added: 14 Total: 14
Next up, romantic orientation. The longest one is Desinoromantic, at 14 characters. This is also great, because since it means just "liking" someone and not really experiencing romantic attraction, it allows us to plop another one on top to determine which gender you're attracted to within this. I believe the best one to add here is Ceteroromantic, which is usually defined as being attracted to non binary people, putting our total for romantic orientation at 28 total characters.
Added: 28 Total: 42
And finally, sexual orientation. Gray-asexual means that you are somewhere inbetween asexual and allosexual. Again, this is great, because not only is it 12 characters, it allows us to add Ceterosexual (ceteroromantic but sexual attraction instead) on top, for 24 characters added for sexual orientation.
Added: 24 Total: 66
So, to summarize: The longest LGBT orientation I can think of right now is:
Transmasculine Desinoromantic Ceteroromantic Gray-asexual Ceterosexual. Anyone out here who identifies as this, good goddamn job. Anyone who thinks of a way to make this longer, hit me up.
Edit: One thing I didnt consider is that to be transmasculine you have to be born a girl. If you were born a guy, transfeminine is 13 characters, making the total 65.
Edit 2: You can also add Intersex and Nonbinary to the Gender portion, making the total +17 = 83