More than that. The MC and the other girl break the rules and sleep together behind their partner's backs. The cherry on top was when the side girl calls the FMC while she's having sex with the MC.
This manga is called ‘Even So, I Still Want to be Your Boyfriend Tomorrow’ , a title that doesn't mislead and is a clear statement of the author's intentions about what is going to happen. The FMC hasn't been too concerned about whether what she does is right or wrong, or what the MC does. They are a couple because the author decided so, not because they demonstrate it throughout the chapters. Tomorrow's chapter may clarify what the FMC wants.