as far as i know there was a big war in which the monster girls turned against the goddess and humans
then the current monster queen and most of the remaining monster girls went on the queen's island to end the war
then they put this protection on the island.
the mc is officially send on the island by some guild to "defeat" the demon queen and pay off his debts
but he isn't the first one and most guys end up in the capital
then there is some ominous evil being beneath the island...
(maybe it's trapped down there)
the rest is breaty much conspiracy theory
my theory is:
that this island is a prison or a exile/safe haven for all the remaining monster girls
made to prevent another war
the queen and the goddess made a deal:
the goddess sends humans to the island as entertainment for the girls (as lover/sex toys)
in return:
the monster girls won't kill anyone anymore
they will stay on the island forever
and the goddess and her angels watch over the island as guards