I think one issue with people thinking others are "faking" is that they don't realize that bisexuality and biromanticism can be two different things - someone can only be romantically interested in people of one sex while being sexually attracted to both. I generally take people at their word for what they self-identify as because generally the issue of whether they're "faking" is not actually a big deal.

People DEFINITELY fake it for all sorts of reasons, but I agree with Pestilence, people don't realize bisexuality and biromanticism can be different. I know several people who are bisexual but not biromantic (or the other way around) and it took them a while to realize that they were separate things. Generally I think of bi as liking both men and women, not necessarily anyone in-between but that varies. It's hard to draw the line between pansexual and bisexual when you start talking about attraction to trans and intersexed folk because a lot of bi people like anyone regardless of their sex/gender but don't identify as pan because they still CARE what sex their partner is, while pansexual folk usually don't care at all. I think it all comes down to what you'd prefer to call yourself, it's just impossible to make hard lines in regards to sexuality/romanticism.

I think bisexuality and pansexuality are sadly misunderstood. I think even people who identify themselves as bi or pan sometimes don't really understand it, especially in the case of bisexuality. I think there are girls who call themselves "bi" because it's the thing to do and guys think it's hot and I'm sure there's other reasons someone might claim to be bisexual when they aren't truly. I think there's people who call themselves bi in a way that's misleading, which is where the bisexual vs. biromantic comes into play because some people like to sleep with or fool around with people of the same sex but ultimately want to settle down and have relationships only with the opposite sex which has resulted in a number of broken hearts. I think there's people who might truly be bi but because it's easier to be in the closet and be in hetero relationships they won't really consider a serious relationship with someone of the same sex, even though they might be romantically and sexually attracted to them. I think this might actually be more common than people "faking" it. I think that coming out is hard, being in non-traditional relationships is hard, being anything other than straight can be really difficult. For most of us, we're raised with the assumption that we're straight. We're raised with the expectation that we will grow up, meet and fall in love with a person of the opposite sex, marry them and have children. Sometimes, the ideal of the wedding and the picket fence and the 2 car garage and 2.5 kids can be hard to give up, especially for those of us who really bought into that fantasy when we were little kids playing with Barbie and G.I. Joe. I think the stigma is even harder for guys to shake since so many are brought up with such a negative image of male same-sex relationships. For many men, being with another man challenges their masculinity. I think that being bisexual can be very difficult because the option to be with a member of the opposite sex is still on the table. For those of us who are gay, our option is to come out and be with someone we really want and are attracted to, even if giving up that picket fence fantasy is hard, stay in the closet and live a secret life or totally deny ourselves and be unhappy. For bi- or pan-sexuals, they can be attracted to members of the opposite sex and be happily in relationships with them so it's possible for them to appear straight to the world and still be in a relationship with someone they want.
I think there's also a spectrum at work in pan and bisexuality. Someone might be pansexual in the sense that they have the potential to be attracted to people of all genders and sexes but they can still have preferences or "favorites" if you will. They might still have a "type" of person they are most attracted to and those "type" of people might tend to be one gender more often than another. I know a bisexual girl who tends to be more attracted to women than men but doesn't write men off altogether.
As for what "qualifies" as bi, I'll generally accept anyone's self-labeling. If they say they're bi, then they are. As for the last question, I'm not bi, I'm a lesbian. I've known I was gay since I was about 18, though I was questioning for a while before that and looking back, the signs were all there and I probably should have caught on sooner. I came out when I was 20.