Gotta re-read what you quoted, Kuli.  I already said that.  The *one* pertinent piece of information being conveyed by ID'ing as straight or gay is which gender you date.  When you hear someone is bi, that's more iffy.  Maybe they feel some degree of attraction (between 1 and 99) for one gender and the difference for the other, but you have no idea what that breakdown is.  And just gauging by how these threads tend to go.... bi people seem to prefer no one asks, thinks about it, or talks about it.
		
		
	 
Bi people react that way because you have people like the idiots here who don't want to hear about anything complex, they just want a simple label so they can put the person in a pigeonhole and think they understand.  You have people who think sexuality is fixed from the womb and everything else is just pretending or lies.  You have people who, when any serious response is offered, turn it into an opportunity for insults all over the place.  Almost no one wants to hear about innate attraction v learned attraction, or even different kinds of attraction unless they can make it a dismissal about "aesthetic", which is a way to ignore the fact that it 
is complex.
Gays accept easily that some guys like muscled, some like skinny, some like hairy, and so on, but the moment liking women comes up, the acceptance of the differences goes out the window.  In our brains, though, where the wiring is, there's really no difference; preferences or attraction or orientation or whatever you want to call them aren't really things we choose:  some are inborn, many are learned, some are adopted because of peer pressure and become self-trained.
If you're hitting on a guy and he tells you he's bi, that's telling you the same thing that hearing he's gay would:  he's up for possible further interaction.  It also tells you that if you were female, he might also be up for further interaction.  Whether walking down the street he ogles three times as many chicks as dudes isn't terribly important, because he's already said he's open to you, and that's all that matters.  Neither label tells you if he likes NASCAR or ballet or American football or scuba diving or loud cars or big trucks or shopping or decorating or what, but both labels tell you that you have a green light.
As I've said before, I've met guys who came out as bi but later on realized that all their attraction to females was learned, and so conceded they were actually gay.  But when they said they were bi, they actually were, because they had the attraction to both; and when they later said they were gay, they actually were because the learned attraction had faded.  And as I've also said, some of us never figure out how much was learned/trained/brainwashed, but that really doesn't matter because even if 
you knew, you should treat them with respect and honor where they are.
But since there are so many who would rather judge than honor, people get defensive.  People also call those who would rather judge on their bullshit, because it doesn't matter if you're coming from a perspective of the First Apostolic Spirit-Filled Community Church of Jesus the Man or wherever, judging is judging.