There is an interesting medical condition called CAIS: Complete Androgen Insensitivity Syndrome. People with this condition are classified as female at birth, they have normal female genitals and develop female secondary characteristics such as breasts, hips etc in a normal way during puberty. But these women do not menstruate, because instead of ovaries and a uterus they have testes (undescended in the abdomen) and an XY sex chromosome.
They are genetically male, but have a mutation that makes them completely insensitive to androgens (e.g. testosterone) so instead of developing male characteristics they develop female ones based on the estrogen produced by the male adrenal glans (all normal males produce estrogen, it's just that testosterone is the dominate hormone).
So aside from modern medical science, a woman with CAIS is indistinguishable from a genetic woman aside from the fact that she can't have children, which happens all the time to genetic women anyway. If we include modern medical science, then women with CAIS are indistinguishable from MTF transwomen.
Here's a link to read more. I find CAIS very relevant to the "would you date a trans" discussion because if they are indistinguishable from a genetic women, does it really matter?
http://en.wikipedia.org/wiki/Androgen_insensitivity_syndrome
