Houkago no Yuutousei.

Our protagonist is a helper at his uncle's inn on an island. On this island being in a relationship is frowned upon, but the girls still have lustful tendencies... Two honor students decide that they are going to use our protagonist as an outlet for their sexual frustration! Anytime the girls wanted sex they got it. These were supposed to be wholesome and clean honor students, not sex crazed monsters!