Hahahah. _ I am not saying that people are not bisexual. "Everyone" is probably an over statement. However it can be argued over 50% of people are biologically bisexual.
The complications about speaking about bisexuality is that gender can be a bit hard to define if you don't latch on to a specific thing to distinish themselves apart.
For example, one could say that they will define a male as having XY chromosomes and a woman as having XX chromosomes. This can be convoluted however because it asumes that abnormalities do not occur during birth and the child results in being XXX or XXY.
Another example of what to latch onto to define gender is the genitalia. This becomes difficult again however because 1/500 births are hermaphradites.
The point I am trying to make is that you are over simplifying the subject. It is much more scientifically and socially entangled then it looks at first glance.