It would be completely an okay age gap if the MC wasn't a minor? Like, not everyone is going to be able to look past that and enjoy playing a game where you have to read a romance about an underage girl and a grown adult.
eh not that much, this obsession with the age 18 to consider someone an adult is more of a US thing in general , its legal to get married by 16 in some places so it really aint that weird
