The one she chose to impregnate her was none other than her own father...

"Oh Saintess, sacred thou. For the sake of confronting the demon lord, I call upon thee to conceive a child with thy dearest beloved."
On that day, I, the daughter of a lone priest, received my calling as the Saintess. My body was bestowed with un-aging youth and beauty and a power worthy of God. Many witnessed the divine oracle's proclamation that day, so none could doubt it.
Being a smart girl with but a very strange mind, I felt a little nervous, but it was a terribly joyous occasion for me.
The saintess raises heroes, for she is the life-giving womb... Indeed, God recognizes her as the most immaculate womb in this world. And I, I am a strange girl who rejoiced at such a duty, so perhaps this is why I was left with such a warning.
"...Thoust holds love too deep. Even I may not stop it. Beware not to let it stray too far."
I know that already. I intend to respect my beloved's wishes, after all.
"Good morning... Father."
And it is he, who was but a low-ranking priest, who is my dearest beloved.
The one who sired this now flawless womb- my father.