[quote=@Little Bird]
Defining a character's sexuality doesn't necessarily equate to an RP being erotic though. It's a legitimate element to include in environments wherein romance is likely to emerge at any degree; frankly, any brand of high school/college setting has a door open for that sort of stuff. I just view at as supplementary to the character's personality. Really just depends on how it's played out in practice. 
[/quote]
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.