There are no romance locks in the game yet.
Choosing Aeran's romance flag sets the continuity for your player character having feelings for him. This is necessary ONLY if you want to pursue any of his romance content. It does not lock you out from romancing or pursuing other characters. (For example, you can flag his romance in Episode 1, but do an intimacy scene with a completely different character in Episode 2. This may have consequences for your relationship with him, but it is something you can do).