Sex realists share a belief that gametes/chromosomes/sry/other unobservable immutable biologic factor solely impact situations where presumed/expected observable sex categories demonstrably and inexorably affect life.
For the vast majority of people, there is an alignment between gametes/chromosomes/sry/other unobservable immutable biologic factor and presumed/expected observable sex category. But it is not 100%.
Sex realist beliefs cannot (or will not) acknowledge the minority where their presumed/expected observable sex category does not align with gametes/chromosomes/sry/other unobservable immutable biologic factor.
I am not sex realist because I acknowledge this very small minority of people who do not align 'biologically' and 'observably.' I also possess a long history as such a person ("regardless of clothes and stereotypes"), which influences my beliefs away from those of sex realists.
The exchange with @BonfireLady above helped me think some stuff through on this sex realist vs trans acceptance topic.