I believe you misunderstand the story. It was a story about the MC growing up. We saw whether or not MC grew up and side girls like Cindy, Isla, Grace, Giselle was only there for check it. Grace was represent incest taboo relationship, Isla was another man's wife and another taboo, Giselle was his therapist and professional taboo, Cindy was around his age, everything was okay to have a relationship and she was cheating taboo. Every single girl was represent something for MC. What was the first sin of MC, he was in relationship with his mom who was incest, another man's wife, have medical condition and that costed us Mia in the first game. In this game story checks every mistake you had done, one by one. First checks it with Cindy, was it the cheating MC drawn to cheating. If MC goes there, it is game over for him we learn he did not outgrown his past mistakes. If he rejects Cindy, next target was Grace. We are checking was it incest that drawn MC in, if you refuse. Next checkpoint was Isla another man's wife, a taboo of conquering someone else's wife. It was so important, there is a choice between Mia and her in the game. Then lastly, Giselle ethical taboo. Giselle was ethically bound to not have a relationship with, like as the medical condition of MC's mom. Giselle story escalates too quickly just like his relationship with his mom because of her medical condition. We checked was MC drawn in because what he did was ethical taboo. Then mom shows up at the end, we ask did MC do all of it because who Laura was because we stripped Laura from everything incest with Grace, cheating with Cindy, conquering with Isla, ethical taboo with Giselle. Only thing to check was it Laura all along. There was a reason, she came to MC so strong in that stall. It was his final test. At this point, if he manages to refuse her. That means he repented the sin he committed in the first game. I believe it was fairly obvious with MC's Grace dream when he ask were you testing me, Grace replied no you were testing yourself.
After MC defeated every demon he had, what he left was pure love which was pointed out over and over again. That is the point we are asking, is MC gonna move on or stay? Both option was represents love and nothing else. If he chose to move on story takes us to new place, MC and Sadie buy a new house also Mia was moving on with someone new. If he chose to stay, he can be with Mia only and only for his love. If he was with her because she was his sister, he could have gone for his mom or Grace; if he was with her because she was someone else's gf, he could have gone for his mom or Isla; if he was with her because it was wrong, he could have gone for his mom or Giselle; if he was with her because she was beautiful, he could have gone for his mom or Cindy but he did not, he chose Mia because who she was, only for their love. Going back to their old home, represents they stayed the same but outgrown their demons together.
So any side girl ending could be bad ending because any of them would mean MC succumbed to his old sin. If they ever add endings with side girls, you should expect a sad ending. In either, Sadie&Mia , epilogues everyone have happy endings because they repented. It even shows how Vivian became decent human being, Giselle found right path etc. If MC goes for anyone else we probably will see that they lost the bar, Vivian and Grace split up, Isla and Cindy spiraled out of grief, Giselle probably dead or divorced or lost her license, mom lost her path or stuck in unhappy marriage etc. I also liked reunion ending, even if MC succumbed to his old sin it shows he still will have a chance to repent. He is only incorrigible if he did not change at all and goes for everyone. Then he is destined to be left alone.
I feel like I tried to explain a poem in a literature class