Her rejection wasn't really from her heart. It was just to make ML feel at peace. She liked how SML was more like her. Otherwise why'd she marry him just because he asked? She wasn't that old or ugly or poor, so why'd she? Because he was an attraction, something new, something exciting and most importantly he was different than ML even if it were to last temporarily.
It was total mess, all of it.
I wouldn't really suggest anyone to watch this drama but your query was so specific that only 3rd charm came to my mind.