Spoiler
I think he will only accept the fl and not the one with the one night stand as a wife. When I read the novel it didnt really matter to me that he have sex with the other girl because the story really focus on him getting stronger and not romance. They literally are not that close because there are lots of problems and hardship that the ml needs to solvw so their relationship didnt really bloom that much. And when he have sex with her they are really became official just a month ago as a couple. And he became stronger when he have sex with her (the girl he have sex with her that is not the fl is like the ice goddess, he have the hardtike getting close with her when she cant control her magic like ability) and when he have sex with her, he absorbed all her power and combined it with his darkness like power. And became even more stronger. I dont know what happens next because the chapter after the other girl left is not yet translated
I hope the manga doesn't go on the same path as the novel!!! Maybe they can skip some scenes instead???