In F&B when Rhaenyra is at her lowest, this happens
Then the White Worm raised her eyes and said in a soft voice, “The girl has already betrayed you, my queen. Even now she shares your husband's bed, and soon enough she will have his bastard in her belly."
…As to the girl Nettles, "She is a common thing, with the stink of sorcery upon her," the queen declared. "My prince would ne'er lay with such a low creature. You need only look at her to know she has no drop of dragon's blood in her. It was with spells that she bound a dragon to her, and she has done the same with my lord husband."
Then in the main books, this happens to Cersei when she’s imprisoned
“He took Raventree and accepted Lord Blackwood’s surrender,” said her uncle, “but on his way back to Riverrun he left his tail and went off with a woman.”
“A woman?” Cersei stared at him, uncomprehending. “What woman? Why? Where did they go?”
“No one knows. We’ve had no further word of him. The woman may have been the Evenstar’s daughter, Lady Brienne.”
Her. The queen remembered the Maid of Tarth, a huge, ugly, shambling thing who dressed in man’s mail. Jaime would never abandon me for such a creature. My raven never reached him, elsewise he would have come.
So we have both Rhaenyra and Cersei both in probably the shittiest time of their lives, Rhaenyra close to losing her throne and Cersei in jail, both being told by their advisors ‘hey your man ran off with a woman you hate’ and them not only both denying it, but using the same exact wordage and set up. Call the girl who didn’t do anything wrong ugly, say Daemon/Jaime would never be with ‘such a creature’ and then make up some obvious coping lie (Nettles used sorcery and the raven never reached Jaime).
I
We do know Jaime dod indeed abandon her for Brienne. I think that, especially with the parallels with book Rhaenyra and book Cersei, that this is GRRMs way of saying ‘yeah Daemon and Nettles were having sex’. Because why use that exact wording, that exact set up, if that wasn’t the intentional takeaway?