Sexuality is almost always shades of grey, and this is coming from someone who identifies as straight. There are those who pretend for attention sure, but personally people who pretend to be asexual because they suck at relationships bother me more. There are plenty of real bisexuals out there.
Sexuality just doesn't work in a way that always 100% one or the other. That makes no sense at all. Firstly theres both emotional and physical attraction, emotional attraction is essentially genderless and your physical attraction ties it into one gender, but these bindings arent solid, they can be broken. Unless your attraction is always entirely shallowly physical its silly to say that you could never in a million years get with someone of the same sex. Even if we're just focusing on physical looks though, people of one gender can look like the opposite gender to what they are. I've seen men that look pretty, and women that look handsome. and thats not even taking into account trans people.
Personally, I believe most of us have a gender that the vast majority of our attraction is aimed towards, to the point where either we don't see the possibilities of a relationship with the same sex, or we see it but they are nowhere near strong enough to act upon. So we pick the label closest to what we are, straight or gay. Bisexuals simply have prominent attractions for both genders so they are somewhere in the middle of that scale.
To strenghten my point, I have never met a bisexual who i've discussed it with and they have had total 50/50 attraction to both genders. Usually it seems to be 40/60 or sometimes goes as far as 10/90. Another way of seeing how sexuality just doesn't work in black and white.
