Not long after Subaru returns from Vollachia, he strikes a business deal between the Emilia and Anastasia camps. And a part of it includes him visiting the Anastasia camp rather frequently. During one of these visits, Emilia can't help but feel down due to her own inability to confess to him. She fails to realize what her knight and rival have been getting up to for the past month.
Subaru wished he didn't have to do this. But he had to. He was in too deep to back out now, and he doubted anyone would believe him if he told them the truth. After all, people wouldn't respond kindly to the fact that he'd been having sex with a Sin Archbishop for the past few months.