Masaomi-san was like the proverbial Venus Fly Trap ... once Marie had reached the "point of no return", there was no turning back. And in the end, despite her initial hesitations and protests, it turned into a case of "Hey Mikey, I Think She Likes It."
...Well, at least he offered to let her run away instead of 'forcing' her to go with him?
Different story when they're already at the hotel though, I suppose.
That ending...Well, In a way, her sister isn't wrong. Maybe one day she'll have a baby of her own.