Because it's been proven, sir.
A quick search found a paper called
loveual Orientation in Women with Classical or Non-classical Congenital Adrenal Hyperplasia as a Function of Degree of Prenatal Androgen Excess.
If Heino F L Meyer-Bahlburg, Curtis Dolezal, Susan W Baker and Maria I New can believe in biloveuality enough that they can write a 15 page paper on how an enzyme deficiency is correlated to emasculation in females (and a suggested correlation to biloveuality), so can I.
Believe it or not, despite what your parents/teacher/evening news/religion tells you, not only has it's non-existence not been proven, but it's generally accepted among the scientific community that it
does exist.