They didn't do it . It's possibly the flashback of when they jerked off together back in high school.
im pretty sure that flashback was his r*pe fantasy from the chapter where they ask about his first love
Well , Nobody actually know if they actually jerked off together in highschool or not . Since , past hasn't been shown yet . Seeing these tidbits of past I think they did . That's why I added "Possibly" in my comment since its one of the possibilities that can't be ignored. Because the way MC & ml act , Sth must have happened in the past . But one thing is for sure they didn't have sex during their highschool days .
about jerkoff we know it but in this chap the scene is going all the ways and bit dejavu so just like did they do it and pretending nothing happens or the seme do it with someone else but why suddenly remember when he do it with the uke but never mind that just opinion （￣へ￣）
try rereading the end of chapter 17. the panels in chapter 24 where it looks like dejavu (the flashbacks) are from seme's dreams in high school.
chapter 24 is the first time they actually did it (seme also says 'to think i'd only get to taste you 10 years later' which i think confirms that they didn't do it in hs.
i think that the uke and seme ever do it once when they in the school so is like dejavu? or what (๑•ㅂ•)و✧