yes, he was. and here i am thinking they're might be related as a biological father and a son which makes this even more fucked up than ever. ┑(￣Д ￣)┍
Your theory is actually incorrect because sensei released a timeline of Norihiko and it says his first experience with Ikuro's mother was when he was 15. Ikuro was 5 at the time so he was already alive before Norihiko and the mother started getting it on.
http://www.mangago.zone/read-manga/psychedelics/mi/manga/psychedelics/c011/pg-10/
Heres the link. It also has some real specifics about Norihiko's life and his feelings towards Ikuro. Sensei is so thorough!
No. He said(while fisting the woman) "Ikuro came out of here when he was born."
http://www.mangago.zone/read-manga/tourou_no_ori/mf/324043/Ch3/pg-1/
I'm not sure i've understood correctly but Norihiko was also having sex with the mother when Ikuro was little, no ? I wish he wasn't tho ...