It should be noted that this graph does not conform to the standard bell curve that is used to describe most characteristics of humanity, so that one concludes that some unusual feature of life is at work and producing a distortion. It could be that another labeling system might make clear the reason for this. For instance, if one labeled the extremes homosexual and heterosexual and the middle bisexual or asexual or both, this would also allow placing all members of society and would produce a bell curve, though the bell would be displaced towards the heterosexual end like this:
This reflection makes clear the reason for the unusual shape of the curve and identifies the source of the distortion as ideology. For more on this subject read this.