The opera "Carmen", but with polyamory:
Act 1: Micaëla and Don José fall in love and decide to get married. Don José and Carmen fall in love, which is fine with everyone's established boundaries.
Act 2: Carmen and Escamillo fall in love. (Lots of dancing!)
Act 3: Don José is a bit jealous, the three of them meet and talk it out. They find ways to reassure one another better.
Act 4: Carmen's feelings for Don José change, and they negotiate a tricky relationship transition. Everyone gets tapas together afterwards.
@blinry that reminds me of a manga that I once read where they resolved the love triangle with a surprise poly relationship, and I was like "Huh, didn't expect that" XD
@lislis Thanks for digging! :)
@blinry I also found out the author has a new series, so THANK YOU!
@blinry That required some digging, it's Luminous=Blue by Iwami Kiyoko. It's also yuri, I should probably mention that