Eh, it's a bit late now, but I'll post a brief summary for anyone who happens to revisit this, or finds it somehow.
Someone recently died at the office lady's (OL's) company at around the same age as her (30s), which made her reevaluate her life and realize that despite excelling in her job and taking care of herself and etc, she's completely neglected her love life and sexual desires, and resolves to focus more on them. The protagonist lives next to her in the same crappy apartment, he only has a part time job and spends a lot of time jerking off to porn. Since the walls of the apartment are thin, OL hears the videos he watches, but because she doesn't really know much about that kind of thing, she mistakes them for the sounds of women, and seems to think the protagonist constantly brings different women to his place for sex. She tells the protagonist all of this one morning, and asks him if he'll make her one of the many women he's slept with, and he decides to take advantage of the misunderstanding.
OL comes over to his place that night, they have sex and he slowly converts her into a sex loving maniac over the course of a month, yada yada. In the final scene, the OL mentions that she doesn't hear voices of other women anymore, and she assumes this means that he's chosen her, insisting that they get married, and that she'll use her tens of millions of yen (hundreds of thousands of dollars) that she saved by living frugally to buy them a nicer place to live together. The protagonist agrees, and makes an internal vow to improve himself in order to be worthy of her.
It'd be nice if someone decided to properly TL this, it's pretty great. I'd do it myself if I had the patience for CG sets... hah hah.