<Snipped quote by Little Bird>
I disagree that it adds anything useful to supplement the character at all, because I see it as being like the corpo-pandering that says stuff like "Our game has a GAY character!" but then has no answer for "Okay, but what about this character makes them relatable, fun to play, or endearing to the audience?" In 90% of character interactions it's never going to be relevant, and it's the kind of thing that can be obvious without being stated---if a character's commentary or internal monologue notes their attraction to other characters, or two characters develop romantic chemistry through interaction, then their preferences are self-evident.
The only thing I can see it being useful for in a romance-focused RP (which is really rare outside of 1x1) is to tell people at a glance who they SHOULDN'T have their PCs develop relationships with because there will be no attraction present---which I would think would ruin half the premise of a "will they won't they" plot line because you already know the answer from the beginning of the game.
Having a bunch of goofy teens trip over themselves to blurt out their crushes, then get shot down because their crush isn't into them, is probably where at least 50% of the interesting character drama/development would come from in such a story. So plastering a label over everyone's head at the start to immediately take some of that "would he/she/etc accept me?" element away is probably why I've seen those RPs fall off even faster than the hordes of generic Medieval Anime Fantasy games.
Not sure about the "pandering" aspect. This is RPing. Not Hollywood. Openly allowing people to play a character of a specific orientatiom doesn't equate to "Hey we have a gay character" the way it migbt in a pre-scripted movie or show. Even if it was that way, by your outlook, wpuld it not be just as much of a pander to play a character as bi/gay or build up to a come out without explicitly mentioning it OOC?
Having it openly labeled doesn't change how well a person plays the character either, and just because the people in the RP are aware of it doesn't mean the characters need to be. Rarely am I venturing into an RP and looking to throw people for any kind of loop based off of undisclosed information; not in situations like that. In my personal experience, the RPs wherein sexuality has a spot on the CS, things like relationships are planned and plotted (mostly) ahead of time anyways. So the "will they, won't they" isn't a factor OOC... usually. So being open about which way one's character(s) swing(s) becomes useful in ironing those matters out.
I agree to the extent that a characters' personality should be able to stand without leaning on sexual orientation. But I don't think it inherently detracts from an RP if the option to specify it is present in the CS. I'm sure as Hell not gonna abandon an RP that I find appealing over such a minute detail of the process. But that's why it's your pet peeve and not mine.