By Beth OrensBack in 1997, a transwoman going by the name of Starchild0 posted an article on the alt.support.sexreassign newsgroup. In it, she proposed that the two sexual orientations we ordinarily refer to - heterosexual and homosexual - are in fact special cases of four real sexual orientations:
Clearly, in the case of a person who is not gender dysphoric, these four orientations will collapse into the two standard ones, and any ambisexuality will collapse into standard bisexuality. It is only in the case of those who transition from one gender to another that the full spectrum of orientations can be seen.
Suppose a man is attracted to women. Is this because he is truly attracted to females, or is it that he is attracted to members of the opposite sex? In most cases, there is no practical difference between the two. But if this man is gender dysphoric, and transitions to female, the difference becomes highly significant. If he was attracted to females, that attraction will manifest itself as lesbianism after transition. If his attraction was to the opposite sex, her attraction post-transition will be heterosexual in nature.
In Figure 1, I have demonstrated this. The square represents the four way spectrum of orientation, and the two lines are the projections into a single dimension of that spectrum for males and for females. An individual whose sexual orientation is located where the red dot appears would manifest as entirely heterosexual if male, but as bisexual tending towards lesbian if female. Thus, when a transman or a transwoman appears to be "changing orientations" following transition, it may be that nothing of the sort is happening.