"Mom, how could you?"
"How could I what, dear?"
"How could you cheat on Dad?"
"What would make you say that, Ronniekins?"
"I was at St. Mungo's for the team physical. And they did a regular blood test, and it showed I was related to someone in the hospital. I thought maybe they just meant the blood from when Dad got bitten by Nagini, but no, it was someone in the hospital."
"Oh, dear."
"Mom, tell me. Tell me now!"
"Don't shout at me, Ronald. No matter what, you are still my son."
"Fine, but just tell me."
"You know how much we wanted a girl, right?"
"Yes."
"Well, your father and I were at Hogwarts for a parents' weekend before Bill went, when all of a sudden Sybill Trelawney went into a trance and spoke in an eerie voice. She said: 'To gain the king's true treasure, he must send himself to other pastures. The golden one will provide the impetus for change' Well, Arthur is the king, and Guinevere his treasure, so we figured I needed to sleep with someone else in order to get the girl we wanted. And who is more golden than Gilderoy?"
"You mean I only exist because of a prophecy?"
"Yes, dear."
"Bloody hell! Now I know how Harry feels."
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Prompt from HermioneDiggory: 4. A Weasley is horrified to discover that he/she is the lovechild of Molly and Gilderoy Lockhart.
Many thanks to Southern_Witch_69 for the beta!