I like the acronym "LGBTQ"--Lesbian, Gay, Bisexual, Trans*, Queer. I think lesbian/gay could be condensed into one, since most lesbians don't mind being called just "gay", and then it's be GBTQ.
As for pan/omni-sexuality, those can both be under the blanket term "bisexual", which has recently fallen out of its original definition of liking the two binary genders. "Queer" can refer to demisexuals, intersex people, and most anything else that doesn't fit in the other three, since "queer" is also used as a blanket term for the whole community.
Just my view on things. As it stands, LGBTQ works fine in my eyes.
EDIT: On the issues above:
Non-monosexuality is a thing, and sexuality is fluid. People change, so sexuality can change as well. Sometimes it takes a while to figure out who you are attracted to; maybe you thought you were more attracted to women than you presently are, maybe you realize you're attracted to both, many, or neither. And that's how one can have a "different" sexuality when younger. Plus, hormones are kicking in during adolescence, and as it stands heterosexuality is the norm. It makes sense that one might believe and even feel that they're attracted to the opposite sex, before paying more attention to how they feel and the signs they're given.