Question for Truckers: Define the "B" in LGBT

How do you spell bisexual? Yeah, we ought to accept a person's definition of him/herself. But in the abstract, as I discovered last year when trying to explain things gay to an inquiring relative, it's a very slippery term to define.

Case in point: I have a close friend who, after nearly 30 years of marriage and kids, got a divorce so he could have the gay life he always wanted to have. (The kids are cool with it, btw, very supportive, and he and the ex are on good terms now.) He's not interested in relationships with women, only with another man.

I would tend to say he falls under the bisexual category; he says, oh no, I'm definitely gay, always have been, got married because that's what was expected, and I really wanted some kids.

I take him at his word. But then what does it mean to be bisexual? Where would you draw the distinction, Truckers?