I'm trying to find the title of a hentai oneshot I read a long time ago. The story goes something like this:

1. Guy has 'slutty' looking girlfriend (full figured). He makes her orgasm easily, so he is bored with their sex life.
2. He wants a 'pure' looking girl (virgin, slim, flat chested, etc). So he goes looking.
3. He finds a pure girl and tries to get her to have sex with him.
4. She says he is clumsy as hell at sex, essentially rejects him.
5. He goes back to his girlfriend, and she confesses that she orgasms quick because she loves him.
6. He realizes she is his 'pure' girl.

Thanks for your help.

It's in one of the chapters of sister trap called pure girl.

Wow, thank you so much! It was a long shot. Didn't actually expect anyone to know.