You should actually go back and read the mothers explanation yourself. And then go back and read what he says to Kaeha before leaving. She literally says that MC left a curse on her with what he said to her before leaving. She felt the need to create an heir to her sword style because of what the MC directed her to do before he left. She was so in love with him (his fault for leading her on) that she dedicated everything she had, and sacrificed her happiness, in the hopes of seeing him again because in her mind, if she was able to fulfill what he directed her to do, it would make him come back. I'm sure him constantly up and leaving all the time has nothing to do with her erratic behaviour either. The mother even goes on to say that it's entirely possible that the wife and husband were cursed in their own way because they didn't know what Kaeha would do if they didn't give her a child. Hilarious that you think they were all for it. I don't care what the in-lore excuse for all this is. I think the author did Kaeha as a character a big disservice. That's my point. You seem to think that I don't want it to be edgy. No, this was just mid to me and felt flat. You can give lore-based reasoning all you want, but it still comes off as exaggerated with the whole seeking an heir out of wedlock weirdo shit just because MC said for her to pass her skills to an heir so that heir could teach him when he comes back. Like, are you serious? Her entire character is going to be this tunnel vision husk of a person who dedicates her entire life to a single request from the MC? Yeah, what a masterpiece. So unique. Definitely haven't seen that tired old trope in manga/anime before.
Don't try to tell me that I want this to be another "slow life isekai", lmao. And get out of here with the harem nonsense. You can disagree with me all you want, but don't try to reframe my comment with bullshit about harems and generic slow life shit just because I don't think this specific side arc in an otherwise good manga is some bastion of literature like you do. Didn't like it, felt overly dramatic and exgaggerated, and it was. Long story long, it wasn't for me, and I'm more interested in where the MC goes after this, because everything else has been great so far.