Yes you're right, not just this doujin but everyone on this site have read child pornography if they have read a yaoi with a 17 year old or younger :)
Child pornography is not only limited to an mid adolescent (Yuji) and an adult (Gojo) but to anyone that is under the age of 18 that are engaging in sexual acts, so if one of the characters in any manga is under the of 18 and are engaged in sexual activities then it is child pornography.
Good examples are:
Hidoku Shinaide - Nemugasa is 17 at the beginning of the story and is blackmailed and raped by the other party.
Yarichin Bitch Club - Tamura and Yuri are 17 and are depicted having sex.
Caste Heaven - Azusa and Karino is 17 and are often depicted having sex together.
Even if it doesn't seem like child pornography, by the law it is so if we read yaoi then we're guilty too as we have to at least read one yaoi in a high school setting.
This is why you don't see any BL manhwa/webtoons having one of the main character under the age of 18, the biggest example is, 'Even if you don't love me', the author didn't want to show the sex scene of the main characters as Seungeon (ml) is 17 years old, while it was fine for Hyungseo's sex scene as he is 18 years old (returning student).
Illiterate hoes. Go read the law or something. Child Pornography is bad, its a crime. Even if it's fictional, it's still a depiction of a minor and a grown adult. I know some people are gonna say "you do nothing to help with real life pedo" mf at least I know what's right and wrong. Please grow tf up and change yall mindset. A drawing of a minor getting fucked by an adult ain't hot, its disgusting and should be stoppef.