When Kousuke and I were relaxing in the room, since we both had the day off from the gym, we had unprotected sex.

While Kousuke and I were relaxing in the room, we both happened to have the day off from the gym, so we had unprotected sex.
When Kousuke and I were relaxing in the room—one day we both happened not to need to go to the gym—we engaged in unprotected sexual behavior.