A bisexual is someone who exhibits bisexuality.
Bisexuality is one of the more commonly misunderstood of all sexualities, next to transsexuality. Some of the most common misconceptions is that it is a passing phase, whether into homosexuality (being gay) or from that into heterosexuality (being straight). Most often bisexual people are considered confused, and wierd, and many misinformed people are convinced that one can only be gay or straight.
You're either straight with bi tendencies or gay with bi tendencies. That is the so-called "bisexuality", just a slight leaning. If you're not bisexual, you're 100% gay/straight. If you're "bisexual" you're only 90% gay/straight, and you should be called that, NOT bi.
These "bisexual" women who live with men prefer males and they'd only want sex with women for fun or for a guy's sake. I don't think they'd get fulfilled being with a woman. Not to mention, some "bisexual" women do it for the attention - it's a trend these days and it's seen as hot by society.
Straight men who have bi leanings are on the straight side and they get fulfilled with the women, so they wouldn't need men, even though they might have slight attraction. Also, since society isn't approval on man-on-man sex, men (who are straight with bi leaning) wouldn't bother admitting they have slight attractions towards other men. So, thus, they will be called straight. And that's a rather correct term.
That is why bisexuality in men doesn't exist, but in women it seems scattered. Women call themselves by very easily, even when having a slightly itty bitty attraction. If homosexuality was more accepted these days and if men weren't so homophobic, even they would be calling themselves bisexual.
Dude: Hon shut up and stop being an attention seeker, you're straight!
Gay guy: I had it with this chick yesterday, she was hot and I sort of liked it. I guess I'm bisexual. But I didn't really...feel...huh...I dunno...I wasn't...uh...um
Straight guy: Dude, you're gay! You didn't feel anything you mean. You're gay! Just because you banged a girl doesn't mean you're bisexual.
Straight guy: I walked and I saw this handsome man. I accidentally touched his bottom, I did feel tingly.
Guy who thinks everything is gay: Bro, you're gay. Ew...come out...you're so gay! OMG!
Straight guy: I'm straight as they come you douche, but we humans are sexual beings, we will feel something. That's why so many men get raped in prison because there is bisexuality among us but that doesn't mean we're fucking bisexuals. We're sexual creatures!
Guy who thinks everything is gay: You're bisexual then!
Straight guy: No I'm straight. I have to be fully fulfilled with a man, but I know I won't be. That's why I'm straight.