Homosexuality, heterosexuality, bisexuality and asexuality (I do think it's a thing, just misrepresented, misunderstood, and inappropriately claimed) are all about which type of partner one is interested in. Demisexuality does not specify these things, but rather the manner in which you seek these partners.

Right: "demisexuality" isn't a sexual orientation; it is, at most, the form that an individual's sexual orientation takes-- the way one's sexuality expresses itself. So, even if we're talking about a same-sex-attracted person, this has no business being included in LGB; yes, the person does... but not the nit-picky sexuality-subheading that "demi" is. No letter for those guys! "LGB" needs to be understood as, and reserved exclusively for, sexual orientations that are same-sex-attracted. So you need to be either homosexual or bisexual to qualify. The end. Everything else is either heterosexuality, asexuality (effectively the same as heterosexuality in this context, due to their mutual lack of SSA), or just the particular spin that an individual's personality puts on their sexual orientation. The latter may or may not be annoying, but either way, it doesn't rise to the level of a sexual orientation.