X-Change 2

Take on the role of Takuya Aihara, the unlucky chemistry student at Miyanomori Private School, who has a tendency to get caught in bizarre chemistry accidents that cause you many problems. When a strange chemical turned you into a girl last year, you had a bizarre string of experiences as you learned "how the other half lived." Now, one year later, you're enjoying your school life with Asuka, your steady girlfriend. But unhappy days are around the corner, as chemistry club member Chisato uses you as a guinea pig for a new experiment, which changes you into a girl once again!

As a girl, you find you're much more popular than before, but with practically everyone around you (both male and female) yearning to have sex with you, it can all get to be too much.

[From Peach Princess]