Ever play some meta games when you continue the story even after you reacted a route end?Not even hidden in this case.
“Stay with me forever.”
1) Stop.
2) Stay with her.
There is no 3). This was an unambiguous all-or-nothing fork.
Heck, it is pretty common to have a true route unlocked after you clear other heroines route.
Some even went for another mile that you need to pick some harmless choice deep in another heroine route.
Some extreme enough that you can't even change that choice even in newer playthrough. The only way to redo the option is to uninstall the game and clean the save data somewhere.
Japanese eroge is some hardcore stuff.