When she falls in love with him she realizes she doesn't want someone experienced who will just dump her afterwards. That being said! I think that Yamada was really shallow. I mean of course Kosuda, the average guy, (like Hedeki-Chobits, Kouta-Elfen Lied) is going to fall in love with a beautiful girl who acts like she's interested. None of the story was even fair to him.
He actually liked Yamada. There is one point where he does question his decision though. He actually wants to make sure he likes Yamada and not just her body. The fact that they have to ask themselves if they really like each other speaks wonders about they're relationship. They are trying to hook up and they know nothing about each other.
I found the sex gods hilarious. It was a good way to keep everything censored by using humor instead being like "we cant show this" so that was funny and cute the way the were incooperated.
Kanejou, kanejou, kanejou. She was really incestual and creepy. Her love for her brother was entertaining but it became too obsessive and stand offish. It was kind of disgusting to watch after awhile and she became a character I was annoyed with very very easily.
Her brother was just annoying. The only reason he was after Yamada was because he couldn't have her. And although I didn't like him what the hell happened to him and Kanejou in the end. They pretty much disappeared along with every other character.
On the subject of sexual guidance Kosuda's sister was pretty brilliant. She was very funny and even though she made some situations awkward she was Kosuda's support the whole show.
Miyano definately should've been the one to end up with Kosuda especially because SHE REALLY LIKES HIM unlike Yamada (in the beginning)
All in all the show was pretty great. It carried elements every romance anime should have and I was pretty happy with it
