I see what you mean, but I'm not entirely sure I agree. In a lot of NTR games, the woman instantly falls for the bull the moment she sees his cock, there’s no build-up at all.
At least in this case, with Azusa, while the corruption happens somewhat quickly, there is still a build up which feels gradual enough for me.
Initially, she's genuinely angry at him, scolds him, and then things happen, he ends up apologizing to her son. A series of interactions and events follow, the rain, the nostalgia of her youth love, his boasting, his lies, her naivety, each contributing to her shift in attitude and finally, their first sexual encounter is basically him raping her while she is internally lamenting to herself how she is a failed mother, but finally losing to the overwhelming power of her orgasm fueled by all her pent up sexual frustration. Something basically broke inside her this night, and her hidden slut was released.
I find the eroticism of this scene really good.
So, while it's not perfect, it's not entirely true to say there is no build up and no gradual corruption. At least, that's my opinion.