Thread of Awesome: THIS! IS!! QUOTEFILE!!!

[classicdrogn]

(Robert) would pester Cersei like an overly enthusiastic golden retriever, give her over the top compliments to placate her vanity, tickle her until she screeched, all on top of having a robust sex life. He had ideas, most nobility did, of what it meant to have a wife. And what it meant to have a mistress. Stannis, of all people, was the one to broach the groundbreaking idea that he was perfectly satisfied with Selyse as a wife whom he treated like how he imagined other men treated a mistress. What he meant was being more demonstrative with affection and enjoying each other’s company. What Robert took that to mean was that he ought to be kinkier with Cersei.

... and that, along with Jaime being kept busy training the reformed Goldcloaks and new Royal Marines and Robert getting more invested in those projects than just drinking and whoring as well, was how Cersei got reverse-NTR'ed.

From Down a Rabbit Hole to Westeros ch18 on AO3. Note that while the linked chapter doesn't have any naughty bits as I recall, it's NSFW in ways typical to the source material, and has some explicit sex on SI-Selyse's wedding night, a Robert/Cersei encounter while the above is being put into practice, Selyse and Stannis again in the immediately following chapter, and potentially more as I'm not even half way through yet.