They used to be lovers, but everything fell apart because of cheating and betrayal.
I'd say it all fell apart mostly due to misunderstandings and manipulation/coercion.
Not to say Sakura didn't do any betrayal or cheating. She most certainly did. However I felt like rereading the director moment with Sakura and realized that the girl is easily manipulated/coerced. Mainly due to her own insecurities.
The director was expertly analyzing her and using those insecurities to his advantage. Getting as far as he did.
With her insecurities during this fuckboi arc (or whatever we call it) they played on that when bashing Shinobu and had Sakura begin to doubt herself. That and all the pushing they did ultimately ended up causing Sakura to just give up as she did. We have to remember that she initially didn't want it. It's just Yuma's aggressiveness towards pouncing on her gave her little time to react and this time there was no outside interference.
Sakura believed Shinobu did it first possibly. Yet she still called out to him for a moment before her mind was lost to the pleasure she felt. It's still a betrayal, there's just a lot more to it than that.
It's a rather complex circumstance.