ML is 12 years old, and FL is 17.
Read this twice.
ML claimed that FL is his bride while some demon claims that she is his bride.
But that demon actually wants the soul of his dead lover in the FL body.
In the end that soul of the demon past lover and the demon itself vanished.
We did not get to see an ending after they got married and when the ML has really grown up.
As it led only till their wedding and the ML has grown a bit older but not all grown up yet.
ML is one sly and perverted little boy that has a way with his words and actions that would make the FL blush and because it is from him it does not raise a red flag.
Hanayome To Futsuma No Kishi