Just cause something happens a lot, in this case the woman falling for the bull instantly, it doesn't mean it's good. But that isn't the issue with this game, at least not in it's entirety. It is fine if you want to make the woman fall to the bull fast, but everything after going at lighting speed too is what harms the writing and game. The mother-son bond being broken so quickly, the shifting of priority between Kujo and Satoshi. Azusa wanting Kujo's baby instantly after the first fuck. All this happens in a couple days, the person that beats up her son regularly gets her to want his baby all cause of a sob story and a few nice words.
Again it is fine if you want to start this way, but your game need to be longer for it. And you need to have your characters interact more, show more scenes, it can even be non-sexual CGs, or scenes of Azusa putting Kujo more and more in the forefront and Saotshi in the back of her heart.
Her reaction to Satoshi calling her a slut was not proportional either. Especially based on what we had seen. She simultaneously "fell" to Kujo on her first day, but also still didn't because her interactions with Satoshi and Satoshi's pov was anemic.
I disagree with you saying that there was build up. We essentially got the following:
Azusa falls for Kujo and ways his baby on the first day - > Continues to not care for Satoshi - > leaves after being called a slut -> Hates Satoshi and an abrupt ending with her.
but given the events of her quick fall on the first day what are we suppose to feel. During the first scene of them fucking she even says something like " I know he is your bully but he is impregnating me right now and I don't care"
It pretty much all comes down to length. The length is just not there for the stories that NTRMAN wishes to tell.
Everything here has already been said I think at one point. The best we can do is move on and hope and pray for the future. I hope NTRMAN finds his footing again in the future.