I'm not going to lie... Erect is an appropriate name for this game in a number of ways.  However, this game is, despite the name, not a pure nukige.  Rather, it is more of a romantic (actually romantic, not just playing at it) charage with high sexual content (about 6 H-scenes in the common route and something like eight to ten in each path).  Now, this is the game that got me paying attention to Astronauts as a company, and it is from Astronauts: Spica, a subsidiary that has so far only produced this game and Aozora Stripe.  

The first thing that stands out about this game is that all the heroines are succubi (in the classic sense).  In the setting, succubi live alongside humans and have chosen to adapt to modern society by finding ways to survive without rape (lol).  An interesting fact of the setting is that when succubi/incubi have children together, their powers actually weaken instead of getting stronger, whereas when they have children with humans the resulting child is more powerful and always a succubus or incubus.  

The protagonist, Kento, is a serious, sincere, and kind-hearted young man who works at his father's flower shop and is learning flower arrangement from his father.  He provides flower arrangements to the local hospital for free as volunteer work, and he is generally the type of guy any community would love to have.  He and Luna, the game's main heroine have a sort of weird platonic relationship going on at the beginning, where she waves to him from her balcony and he waves back on his way to school.  It is cute in a really archaic way, and it is one of the reasons why I actually thought of this game more as a romance than a nukige.

Now, when he happens to encounter Luna in a berserk state in her true form, she drains him (just by touching him) in a way that would turn most people into a mummy.  However, he wakes up shortly after that, just fine and dandy.  This leads to him 'helping' her handle her overwhelming powers as a royal succubus (yes, that way) and attending the local international boarding school, which is a front for succubi and incubi to learn how to live amongst humans.  

Generally speaking, the game is split between culture shock shimoneta jokes, heroines being cute and falling for Kento in classic harem-ge style, and h-scenes born out of the heroines being unable to control their nympho genes in the common route.  The heroine routes, however, are surprisingly thoughtfully written.  The heroines have realistic worries based on their personalities, the parts of them hidden behind the casual eros and natural shimoneta talk come out, and there were actually scenes where I cried throughout the VN because of the intensity of emotion on the characters' parts.

This game has always been a quiet favorite of mine, simply because it took the best of romance, charage, and even nukige and somehow made a package that couldn't quite fit into any of those categories.  The music is actually fairly high quality (above the industry average), and the art style is surprisingly attractive, even now that it looks somewhat dated in retrospect.  For those who want more than casual eros, there are some really strange H-scenes in this game (the girls are succubi, after all), and it never goes down and gets dirty in the emotional sense, at least partly because of the way the girls regard sex in general.  

If you want a game with high sexual content, decent slice of life, shimoneta comedy, and good romance (if not godly) this game is an excellent choice... especially if you like nympho heroines.

Edit: I forgot to properly introduce the heroines.

Lunastia Astyr

The game's main heroine, the current titular head of the Astyr family and a throwback to the early ages of succubi, both in power and her need for life energy.  She has been quietly in love with Kento from afar for quite some time when the story begins.  She has also been surviving by draining flowers bought from his shop of energy to keep herself from going into seizures and randomly seeking life energy from those around her.  A kind-hearted, sensitive, and innocent (other than sex... she is a succubus, after all) girl who single-heartedly loves Kento.  In terms of pure power she is the strongest succubus in existence.  However, her hunger is such that draining a normal human of the entirety of their lifeforce would only give her a few weeks of satiation.  

Ririela Churack

Luna's maid and a member of the Churack family, which has produced retainers for the most powerful succubus clans for some time.  She is unusual amongst succubi for her bashful reactions to sex talk and her unwillingness to show skin if she can avoid it.  She is absolutely devoted to Luna, as Luna is both her best friend and her mistress.  She has a long-standing acquaintance with Kento from buying his flowers to bring to Luna.  For those who want a nearly-normal heroine, she is the best choice.

Euphoria Trinity

The daughter of the director of Trinity Hospital, a local hospital that caters to succubi as well as to the general public.  She works as a nurse while going to school, and she is considered a very reliable senpai.  She has always had a fondness for Kento, having known him from frequent conversations when he brings flower arrangements for the hospital lobby.  While she is powerful, she has a complex born of a physical aspect of her heritage that causes her issues in contact with the opposite sex, even her father (no, it isn't nymphomania, though all the heroines are nympho, technically).  She is definitely a 'ecchi oneesan' type.  

Tiana Gill Lilistia

The daughter of the school's owners, a stiff-necked tsundere ojousama who is a mass of inferiority complexes just waiting to explode.  However, at heart she is deeply passionate, deeply loving, and extremely hard worker.  As a classic tsundere, she tends to say the opposite of what she means more often than she would like, especially with Kento, as his relationship with Luna, whom she considers to be her rival, brings out her competitive side.  

Eris Erotis

The game's sub-heroine/extra heroine.  Called a 'walking violation of vulgarity laws', she heads up the one-member 'pleasure research club' and can generally be found to be linking every word and action to sex and pleasure in one way or another.