"Sexual Orientation and Gender Identity are conferred during the first 12 weeks of gestation, and they are irreversible"
There are some blatant contradictions which Dr Kate O'Hanlan struggles with - but that's really the fault of the constraining binary language; homosexual or heterosexual. In fact, Biology doesn't conform to a strict binary, and the edges blur between those two extremes. Someone in the minority near the divide can be "pushed over" one way or the other - usually towards behaviourally exclusive heterosexuality (regardless of feelings) due to societal pressure, but being abused as a child, or less commonly as an adult, can cause a push the other way. When the trauma is particularly strong, such as rape trauma resulting in a pathological fear of men, or extreme religious pressure to "resist their inner demons", they can become fanatics. The extreme Right-Wing Religious Homophobic politician who's caught with his pants down is now so common it's a cliche.
Or, untraumatised, they can remain happily Bisexual, or Asexual.
A similar case applies when it comes to Gender Identity, that minority who are bigendered can be pushed one way or the other, or remain androgenous or neutrois. How large those minorities are is another matter, but 1 in 3 seems a good estimate.
This accounts for all of the observed facts; that both gender identity and sexual orientation are inherent, congenital, and unchangeable for most intents and purposes, for a few they are not quite, if the person is traumatised enough. For the rest, the trauma does the same amount of damage, but has no other effect whatsoever.