varying states of androgeny that persist through adulthood as their physical "femaleness" is the result of a genetic resistance to androgens causing them to "default" towards female),
Sounds like the family of genetically determined partial to complete androgen insensitivity syndromes. Around gestational week 7*, the mother secretes a burst of male hormones (androgens). In a process called Mullerian differentiation, the great majority of XY fetuses (genotypically male) respond to this and start forming male genitalia, producing a male phenotype.
My mother was a school/psychiatric social worker, who, in the early sixties, was among the first to deal with intersex states -- a term I prefer to what some groups consider politically correct, without, at least for genetically influenced conditions, using more precise terminology. Still, I grew up around people with nonbinary gender/sexuality.
Something that I've run across in some serious forums might work for a story. Mr. and Mrs. Jones are both lovely and loving people, who met as high school athletes. They had a wonderful religious wedding.
After a while, frustrated at her not getting pregnant, they went to a fertility clinic, which did a genetic workup. "Mrs. Jones, I don't have a good way to say this, but you are genotypically male (XY), with either absent or poorly developed internal female organs. In your particular form of androgen insensitivity syndrome, as you demonstrate, one can appear as a beautiful woman. Didn't you wonder, in adolescence, that you had no periods?"
"Everyone thought that was due to my low body fat as an athlete."
*Same week in which arm buds start to develop, thus the first week in which the fetus could attempt the right to bear arms.