oh yeah that one... was it:

right at the end the childhood friend same age romantic interest was told by a different girl that she was pregnant with his child or something along those lines. then he said something about taking responsibility which would imply that they fucked, so the lead girl felt betrayed and decides it's time to hook up with her adoptive father...

then after some pages implying that they are dating the most cursed page in manga of that era where the father complains about being pissed at his daughter's creepy old boyfriend followed by the yet nonexistent of course I know him, he's me meme