As a Bi man myself, it's absolutely everywhere.
I've never been with anyone bisexual that I know of, but I'll take a shot at the "why" question that Allan brought up.
I think if I were to put myself into the shoes of a straight woman (I am a straight woman, so not too hard there) who found herself dating a bisexual man, but not knowing he was bisexual from the get go, and then finding out on a date suddenly that he was bisexual... Let me do kind of a roleplay, here.
The setting: At a popular restaurant after seeing a movie, third date.
Me: So, Allan, I really liked that movie that we just saw.
Allan: I did, too! What did you think about that--
*Suddenly interrupted by an attractive gentleman coming up to the table*
UP (Unidentified person) Hi Allan! Oh my gosh, I've missed seeing you! How have you been? I hope I'm not interrupting your dinner.
Allan: Oh, hi Michael! No, you're not. This is my date, Chanda. Chanda, this is my ex-boyfriend, Michael.
Me: Uh, hello...*Thinking to self, "Ex boyfriend? Is he gay? What does he mean, ex boyfriend?"
Michael: So, Allan, I have to run, but I really enjoyed seeing you. I'll text you later. Nice meeting you, Chanda.
Me: You too.
Allan: *watches him leave*
Allan: So, what were we talking about? Oh, the movie.
Me: He's your ex boyfriend?
Allan: Yes! We dated a month or so ago.
Me: Umm... I... Am not sure what to think. Are you gay? Why are you here with me?
Allan: Oh, I like men and women.
Me: *quiet*
Allan: Are you alright?
Me: Just confused.
So anyway, I think it's a matter that as a straight woman, I wouldn't really know how to identify with someone who swings both ways, especially if I was interested in them. It's not a matter of the bisexual being the problem, but a matter of open mindedness for myself. I would feel insecure, constantly wondering if he preferred certain things that I didn't have (like certain genitalia, or six pack abs, or other more masculine features). He wouldn't be the problem, I would be the problem. By my own insecurities. It would take me quite a while to get used to the idea.