I've read this like years ago and I started looking for it recently but I couldn't find it. I don't know if the author took it down but I really liked the fic.
Married couple chanbaek with four (or so) kids going through some issues after Baek gives birth to their daughter and Chanyeol (a doctor) has an affair with Wen, a coworker. A new year party unveils their "relationship" as Wen kisses Chanyeol in front of his husband when the clock strikes midnight leading up to pure chaos.
Chanyeol sends divorce papers to Baek thinking it would be only fair to set his husband free, which infuriates the latter screaming at him to fight for them instead of giving up ending with them having angry sex.
Other details: Chanyeol has a tattoo dedicated to each of their children and Baek. Baek has only one with Chanyeol's name.
Edit: I will take this down once it's found!
Never take things for granted. This was something Mark had learnt by force after having lost the best thing that had happened to him in years. Or rather, the best relationship he had had in his 23 years of life. He took his boyfriend Jackson, now ex, for granted, and never thought that losing him and moving on was going to be the biggest challenge of all time. But was he really moving on?
No, not without putting a fight first. More like, fighting for whom he should have fought to begin with. It wasn't going to be an easy fit but he wanted his loving (ex) boyfriend back.