There is a medical condition called androgen insensitivity syndrome. Usually, fetuses, phenotypically start out as female, but around 5-6 weeks a gene called SRY gene on the Y chromosome will influence male gonads into testes. The testis will secrete 3 hormones (testosterone, DHT, and AMH) which will guide the development of the male fetus. People with Androgen insentivity syndrome, their androgen receptors do not respond to androgens (DHT, Testosterone), resulting in them taking on the phenotypic appearance of a female.
Most people with this condition tend to be picked up around puberty when the parents become concern that their "daughter's" first menarche is late. So again, they have the outward appearance of a woman with the only exception being that they tend to have sparse or no pubic/axillary hair, as well as, they have hidden testis (that u can't see), and there vagina tends to be a little shallow compared to other girls.
Many individuals are offered surgery to remove their testis and go on hormone replacement therapy to help in developing secondary female characteristics like breast. With that being said, they look, sound, and feel like women and you probably couldn't tell they are biologically male unless they told you and you read up their condition. Would you consider dating one? [Example- the doctor in the video above who graduated from Penn University]