Under the Moon
Uh, ignore the fact that the girl looks five in that picture. It's just a weird angle.
This game is part of the newly budding market for straight naughty girl games in Japan. So much of their smut for girls is boy x boy that suddenly seeing breasts is new and exciting, I guess? Anyway.
This game is kinda like Disgaea erotica. Under the Moon has a supernatural bent: you play as Ashe, the teenage princess of the demon world who's been told by Daddy that it's time to pick a consort and rule the underworld like a big girl. When angels (gasp!) attack your evil palace in order to kidnap you, you and your talking pet kitty cat/guardian escape to earth--but then you find that you're stuck there! Luckily, you're picked up by a pair of twin boys about your age who used to live in the demon world before moving to the human world for better restaurants or something. They prefer the human world to the demon world, but their old alliances to the demon world drive them to agree to protect you until you can find a way back to the demon world. But, hm...you think that one of them could make an AWFUL nice consort, and what better way to find out then to get them in bed? You're looking for prime eternal-dark-husband material, after all.
This game has a few supernatural action scenes with laser beams and magic powers since you're a princess on the run, but it's mostly Ashe trying to find love through a difficult coming-of-age and acceptance-of-responsibility. There's a boy at the human school who seems to take a liking to you, but he's an ANGEL IN DISGUISE (and hence your enemy), but you love him so much what do you do OMG?! And then there's the mysterious older stranger who saves you when the twins fail, but he reveals that, whoops, he's a vampire--but a nice one, and you're the demon princess, so that's really not that weird for you. Your pet kitty can even turn into a handsome young man on earth, and since he's new to a human body and he's been your BFF since he was given to you as a child, you can do some pretty hilarious sexual experimentation with him.
One of the best parts of this game is the fact that you have hot, bitchy twins as love interests, and they a.) both like you and b.) despise each other and hence fight over you. In a special secret ending, you can get a three-way with them where they refuse to talk to each other, so you have to bark orders at them to keep them from fighting over whose turn it is.
Somewhat like Kichiku Megane, Ashe can choose to pursue these boys in a sweet manner or in a bitchy manner. Being sweet is pretty straightforward, but if you act bitchy and play with a boy's feelings then he could get jealous, pissed, or surprisingly kinky. Some boys like you bossing them around--you're the princess, after all. But some get very grabby to prove that they're calling the shots, not you. Some boys are decidedly "dangerous bad boy" types (those twins can get nasty), but others are very non-threatening and can be pursued if you want something tamer. The vampire, for example, is nothing but sweet to you, and you and your pet kitty can spend your days sharing cookies and giggling and occasionally having sex, if you want. There's a pretty broad spectrum of sexual relationships in this game.
Summary: immortal lifespans aside, this game is pretty much populated with hot young people between the ages of 18 and 25 getting involved in a lot of sex and magic powers; it's VERY dirty despite the pretty pictures and sparkles, but all your sex is accompanied by pretty lighting effects and nice music, as odd as that is to say; some of the sex is very edgy and sadistic, and is reflective of the "rape fantasy" mentality of a lot of girl-oriented porn, but you can avoid those characters/paths if you find that kind of thing upsetting; there's a lot of gratuitous girly stuff in the game, like feeding boys strawberries and having them buy you pretty sparkly things, which is pure awesome
Bonus: There's a secret boy from the demon world whom you can unlock, and he's a bit of a pushover and is thus submissive to YOU. You can tie him up and make him into a man if you want, which is pretty hilarious...and you can also drag him into your twin-you-twin sandwich for the logistical nightmare that is four-way sex. It's all pretty insane and amusing.
There aren't many fancy trailers for this game, but this slightly not-safe-for-work one (it's got a bit of licking in it) appropriately shows how pretty the game is.
Under the Moon: the game that proves that despite all the sparkles, girl-oriented smut can be tremendously filthy.