But if Yuuto has a power that opens portals between worlds, since he will be the Demon King, maybe he can go and come back at the same time, especially since his colleagues also need to go, or if they wish to stay, they will still have to report that 9 of them died in the other world.
So now it's proving that Yuuto was going to have a harem if he became the new Demon King. He was going to have to marry the princess, the only one of the old king's lineage, and have his own harem.