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I tried to find it because I'm curious as well, but the only manga I could find that is somewhat similar to your description is this https://www.mangago.zone/read-manga/dude_can_i_some_of_your_dna/
Chap 4-6 of https://www.mangago.zone/read-manga/family_simulation/ is this the one? I also been looking for so I asked and someone told me this one
It’s about I think 2 top students (the mc and ml)and both did science. the ml likes the mc and teases him. And one day while mc was walking in the school he sees the ml masturbate in the science lab and gets turned on so he also masturbated but in the hallway.
And once they finally were together the ml would take the ukes c*m and put it a tube an take with him every where.
Plz help me again