There are stories where the MC and their love interest are already in a relationship. I love the antiques that comes with such stories. Unfortunately, some novels though having established couples from the very start, their relationship are still turbulent and they're still tip-toeing around each other. 

This list will contain the ones I've read, where the pair are already in an established relationship from the beginning of the novel, and their relationship is like an old married couple. Some leniency will be given to stories where the pair have such chemistry but is not yet officially in a relationship just yet. 

Warning(s): Will contain both BL and non-BL novels. Check the genre before proceeding.