"Children wanting marriage and all its responsibilities? I'm not sure there are many of them."
How many adults want marriage and all its responsibilities? How many adults who actually are married actually want all its responsibilities?
"Obviously there are many children who want sex, especially after developing feelings for another human being. Once those feelings are potentially gone for the said person, I'm not sure the desire to have sex totally goes away."
Marriage is about three things.
How many children, developing feelings for another human person, want to live in the same household, do housework together, go out together, have public displays of affection and share a bedroom - the stuff their parents and other adult couples allow them to see - without understanding that inside the shared bedroom, there might be any relevant activities inside their underwear?
How far do children understand the difference between a daughter and a wife?