I'm not quite sure what to make of this story. While I found it interesting to read, I struggled to grasp the logic presented in the narrative, which felt very Japanese to me.
Momoka's character was quite unlikable and often behaved in a reprehensible manner. Her use of the blond woman to intentionally hurt Rin's feelings was incredibly cruel and selfish. Despite Rin's expressions of desire and affection, Momoka continued the relationship solely to inflict pain on her. It was petty and hurtful, especially considering that Rin needed time to develop their relationship. Pushing her into having sex so early on was unfair and disrespectful. If Momoka had shown more patience and understanding, she could have achieved her goals without permanently damaging the relationship. However, as is often the case in these types of manga, the logic behind characters' actions can be quite unique and far from normal human behavior.
In real life, Rin would likely have ended the relationship with Momoka, as her behavior was beyond pale. Momoka did not deserve a happy ending, given the harm she caused to others.