(If they're asexual with one exception, it's Single-Target Sexuality.) A romantic rationalization that removes (or tries to remove) potential squick from a non-standard attraction (Brother Sister Incest, Mayfly December Romance, homosexual for an otherwise heterosexual character or vice versa, etc.) often uses this phrase.
Since the character (usually) acknowledges the unusualness of the potential pairing, the writer gets to assure the audience This is also often an implicit assumption in Slash Fic that tries to pair characters that are generally portrayed as heterosexual (or homosexual—and/or related).
The child was suffering from such a horrible physical defect, but there was no adequate medical specialist in the neighborhood.
The girl's breasts started growing together with penis during the adolescent phase.
The little girl was born having both women's and men's genitals.
Needless to say that it was a totally outstanding event in the small town.
Orlando: A Biography, Virginia Woolf’s celebrated exploration of sexual identity, inspired by her relationship with Vita Sackville-West, is perhaps the best-known literary work on the subject.
Orlando initially begins life as a man, but later “metamorphoses” into a woman while maintaining the same interior persona.