Ah, the heart wrenching insecurities that Fei Yue has towards Bai Yu is relatable, he could withstand the idea of other man spending the night with her wife, but Bai Yu ? Anyone but him.
My heart always supports Fei Yue, it is extremely relatable to see him holding it in and acting like shit's fine while brokenhearted, because you have to suck it up. If you began to show emotions, the emotions to be selfish, jealous, angry, and sad. If you show those off, the one you'll love will leave you. So you have to suck it up.
Regardless, this is a harem. I have no problem with her expanding her harem as it is a meant of building connections and establish power. Promising to be only faithful to one person is, a futile promises. The main characters realizes this very much, Fei Yue knows this bitter feeling well. If you want the position, you'll be lonely.
For centuries, it is women who yearns and waits, on this story, it is men.