Saori is still kind of despicable to me in the end.
Dating a bunch of girls at once and then doing one of your girlfriend's mom's while she's home? And often?
This mom seems a little too accepting, I'd say.
The author even threw in a "Mayu got a new girlfriend, so don't feel bad for her!" Doesn't mean she wasn't cruely mistreated.
It makes you wonder if Mayu knew about the other girls, the fact that she got other girlfriend so fast probably means that she was not so much on Saori and just didn't like that she wanted to end the relationship out of nowhere.
I wonder if it could be considered some kind of ntr, Keiko kind of stole Saori from Mayu and two other girls, although Saori didn't love any of them.
Considering the dialogue on pages 29 & 30: "[...] it's not just you. Saya, Arisu, Mayu, all of them" and the response of Saori being called out as being crazy for actually being in love with someone... Somehow that does not really sound like anyone was looking for a serious relationship there, with Saori simply being a popular player in mutually uncommitted liaisons.
Her actually falling in love for once made her usual way of interacting crumble, forcing her into completely new territory, as she herself states on page 33; coincidentally including a passage from her about how all the previous trysts were not about love but merely about excitement from both sides. Mayu's quick bounce-back also seems to support that line of reasoning (this is not to say that she wasn't disappointed or hurt initially, but her emotional commitment wasn't too deep).