Anyway, contrast with ancient Greece where supposedly, being bisexual was the norm and everyone was just expected to be bi. (This is something I've heard multiple times, but I have to admit, part of me doesn't really believe it, because I've also seen ancient Greeks try to insult each other by calling each other gay or pedophile. If someone knows more about this, I welcome their input.)

You might want to check out the book "Bisexuality in the Ancient World" by Eva Cantarella (covers Rome too, but lots on Greece, plus the latter's practices in this area were deliberately emulated by the former to some extent [cuz snob appeal]).

I'm with you on not really believing the "ancient Greeks = TOTES BI!" canard (much as I like the idea of an officially ambisextrous Hellenistic world); seems a serious oversimplification. First, doesn't seem like we're really talking "Greece" here so much as Sparta and (esp.) Athens; second, as you yourself observed re: Sparta, this institution of male-erotic-activity-with-both-sexes had a social meaning that was very culturally-specific, and doesn't necessarily translate into modern terms (such as what we currently define as "bisexuality") at all.

For example, Athenian "bisexuality"'s M/M component was strictly between men and boys: adult dudes who got with each other (particularly as-- horrors!-- EQUALS) were absolutely NOT part of this institution, and in fact despised. Further, it basically played the role of educational system (believe it or not); since "schools" had yet to be invented, well-born Athenian women were kept completely ignorant (as lifelong illiterate shut-ins), and dads (whose lives revolved around politics, intellectual pursuits, and latter-day "call girls") couldn't be bothered with their sons, boys needed SOMEBODY to provide guidance/supervise their development and maturation... so this is how that happened! Seriously! Grown-ass guy plays mentor/benefactor to boy he gets all dewy-eyed & horny over, till puberty really sets in, the "relationship" dissolves, and both go on to take the man-role with other lads. (Grades 1-12 obviously couldn't be introduced soon enough for these people.)

So, WORLDS away from anything that our pink-purple-and-blue flag stands for, basically.