ぎりぎりパラダイス

GiriGiri Paradise has a rather simple story: the protagonist, a teenager named Keiichi, is having a vacation. Together with three young, pretty girls (initially only friends of his), he goes on a journey, which ends in a small hotel on the sea shore. Exploring the surroundings and meeting local people (some of which are young, pretty girls as well!), Keiichi realizes that this trip is gradually turning into the busiest sex adventure of his life...

[From Moby Games]