In my opinion,
1. MC's grandfather might have asked for the engagement, but in FMC's family, they might not have forced her to get engaged, but she wanted to for the sake of the family.
2. Her family forced her to get engaged and pretended she wanted to, even though she really didn't want to...
Maybe FMC not do it, but if she likes him, she will do whatever she wants to do with him, she not good with men but with MC it’s ok cause she like him but this is so fast to like someone. Arghhh