To be honest, Margarita is a simp, she doesn't like to imagine the MC cheating on her, as she believes we are her special person, the person that will not do anything wrong, the one who is destinated to have a family with her, the person she can't lose
In the end, this reflects Margarita's psychology, she is so traumatized of bad relationships, that she convinced herself that the MC is perfect, she always tries to look things by the bright side, because she will feel pain and depression just to think about the MC not loving her or cheating on her. The only way to enter Margarita's path, is by saying "that it's our destiny to stay together", for you it may be just a romantic phrase, but for margarita is a hope that she won't be alone again, and that she will finally will have someone to love.
In resume: if the MC had a twitch account, margarita would be the simp that donates 100$ every stream just to see if the MC would fall in love with her