People are born one of three genders guys... Hermaphrodite humans are a thing. Now, we're not always born the correct gender, but my point stands. Medically there are three genders.
... Or am I wrong?
Biologically, the system
intends to make males and females, so all the machinery is there to make either (we're all pretty much genetically identical to a large degree). But nature doesn't deal in absolutes. Natural selection only requires that
enough breeders are produced to make the next generation, it doesn't care about whether any individuals is "programmed" correctly. from that perspective it makes sense that you could have someone mostly male, but a few sub-systems got wired the "female" way or vice versa. If there's a spectrum, then I'd say it would be much more likely to be a "jagged" one than a smooth one, since many of the subsystems involved seem fairly independent of each other.
However, even though anything one the spectrum is possible (but because of the vast number of factors, "spectrum" is a complex thing in itself), the system as a whole in evolutionary terms is
pulling towards two poles of male/female, because after all, the entire purpose of the exercise of animal existence is to optimize baby-making.
BTW being somewhere on the spectrum is just much more likely than being somewhere far off it. A sexual attraction to say, chairs, is just far less likely than liking male or female humans purely because the
machinery for being attracted to males or females already exists in the DNA because of evolution. A small chemical spike either way at the right point in fetus development leads to someone developing Sexuality Type A or Sexuality Type B. Most people, even if bi, tend to have a clear preference in preferred gender. Things far from either of those poles just aren't very likely because they haven't been coded for across a span of millions of years.