the future husband fell head over heals in love with the FL. That’s why even he stays with FL and follows what she wants - FL does not want children. In the transmigrated story, he is reborn with all the knowledge of the previous story. He thinks his love is the truest so wants to steal her from her current marriage. -remember that there is no love in the marriage of FL and ML. He stops when he realizes that the FL is truly happy with her marriage. So he is not those me me me type of character. Also, the FL never really loves a person so what does it matter who she stays with.