Once there were some male/female couples and they saw a male/male couple and a female/female couple. Since they almost only ever saw male/female couples around them, they thought that that was the norm, the normal. So they decided to call the male/male- and female/female couples, not the norm, not normal, abnormal. I guess it must have been something like that. It is a statistic thing, i would think, those terms normality and abnormality.