Not exclusive but they trusted each other for the time being to only see each other and figure things out. What's the point of being guilty if genuinely haven't broken that trust ? They may not have been in a explicitly stated traditional relationship but you can tell from chemistry that was a level of trust . That's why for the rest of this webtoon , it's the dude feeling guilty for never following through with that trust. THAT IS CHEATING.
Cheating can only occur within the confines of a relationship or if they both agreed to be exclusive . Because he felt guilty doesn’t make it cheating. If he had slept with taesoo after they actually started dating then you can say it’s cheating. That’s why situationships are totally different from dating because there is nothing stopping you from seeing other people.

What was ch 78 ? YOU SLEPT WITH HIM WHILE YOU WERE IN A RELATIONSHIP??? EUNWOO YOU ARE NOT OK