@pajamawolfie;
I think they just have a mutual respect for each other because it does happen on real life, my life is an example of that (It's true but if you think I'm lying then I can't change your opinion =D ). From the story if they had a better communication on there college days then maybe we can say they could have been together but life is mysterious like that.
About her son; There's a lot of possibilities and one as what you have said also an adopted. But there's also a possibility that he is her legitimate child from her, also, deceased husband as Momoko said "The father that I didn't know."
for the 3rd paragraph of your comment; How she died is really mysterious but for how the story portrayed her there's a possibility that she 'Sacrificed' herself for a child. (Note I quoted it because sacrificing yourself for a child varies per person.) And I also STRONGLY believe that Momoko was adopted after the husband died and there's also a HUGE possibilities that Kanzaki knew the father since he sounds close to Touko's family.
But heck these are just possibilities and speculations on my part. They can even plot twist this and make all the character be connected to each other at this point.
Example: Kanzaki and Touko is close or is an acquaintance of the mother of Haru's mother.
ALSO, There are RAWS? Okay. going to 'Read'.