@john9983 I'm not sure you read the same manga I did.
The sex wasn't gratuitous, and was deeply linked to the story. The same with the religious aspects and the harem. There needed to be three girls, and there needed to be a religious background or the story just wouldn't work. It was four teenagers thrown into a situation where duty, belief, love, and sexuality were all mixed together in odd ways. The entire plot is about how they deal with it, so missing any one of those aspects would have destroyed the story.
Imagine if you found yourself in a situation like one of the girls - you're supposed to compete with your closest friends for the affection of a living god that happens to be an attractive and nice guy. You can't hate them for trying to attract him, but you can't stop yourself feeling jealous. Your insecurities bubble to the surface because you think you're not as worthy/attractive/good as the other two girls. And there's the nagging knowledge in the back of your head that the Rising Dragon festival is approaching, and you don't know what you'll do when it comes.
Or for that matter, you're a guy that shows up on an island and find yourself told you're the main focus of the local religion, and you're presented with three beautiful girls who do anything you say and told you have to choose one. As you get to know them better, you fall in love with all of them. You have to restrain yourself because they will literally follow any command you give them, and that kind of weirds you out. How much of their affection is real, and how much is duty? Then there's the festival coming where you have to choose one... what do you do? How can you make a choice and hurt two people you love?
Honestly, I think it could have used a bit more drama just for realism's sake, but overall I think the storytelling was really good.
(I didn't find the "cult" to be that odd for Shinto, but I'm an outsider so maybe it is. Still, animistic religions are like that. I imagine most Japanese consider monotheism to be strange.)