Referenced with the wiki summaries of the episodes:

Male student Makoto has a crush on female classmate Kotonoha (the girl in the Facebook picture). Another female classmate, Sekai, helps Makoto and Kotonoha get into a relationship. However, Sekai also develops feelings for Makoto.

After dating for a while Makoto feels Kotonoha is boring and starts sleeping with Sekai behind Kotonoha's back. Due to a series of misunderstandings and lies, other classmates accuse Kotonoha of cheating on Makoto even though it's actually the other way around.

During a school festival, Makoto has sex with a third girl, Otome, and then without missing a beat, goes to the dance with Sekai. Meanwhile, Kotonoha is bullied by other classmates for her "fake" relationship with Makoto. At an emotional low from constantly being ignored by Makoto, Kotonoha is taken advantage of by another male classmate; she is essentially raped. The next day, Makoto bumps into Kotonoha and finally officially ends their relationship, leaving her in tears.

One of Sekai's friends, Setsuna, finds out Makoto is cheating on Sekai with Otome and demands that he stop. Makoto's response is to push her down and try to have his way with her. Setsuna eventually relents, but only on the condition that Makoto remain faithful to Sekai afterwards. Back at school, Sekai sees the footage of Makoto having sex with Otome, captured by a hidden camera.

Sekai, shocked by Makoto's betrayal, begins to skip school. During this time, Makoto never attempts to contact her and begins regularly sleeping with yet another classmate. Eventually, even the girls who had been bullying Kotonoha want to see what all the fuss is about with Makoto, and he happily agrees to have sex with them. While all of this is happening, Kotonoha wanders around the city in a nervous breakdown, holding imaginary conversations with Makoto.

Sekai becomes sick one morning and decides she is pregnant. She returns to school and confronts Makoto about it, insisting that he take responsibility. The other girls begin to shun Makoto. While taking a walk, Makoto happens upon Kotonoha, and taking pity on her, he says he will be her boyfriend again.

Sekai finds out that the two of them are back together, and when she goes to demand they break up, it ends with Makoto kissing Kotonoha in front of her. Later that evening, Makoto texts Sekai telling her to get an abortion.

The next day, Sekai goes to Makoto's place … and well, I don't think my crappy summaries really do it justice, so here's the video of the ending:
http://www.youtube.com/watch?v=hIkqFaW_Iic#4m47s
http://www.youtube.com/watch?v=kT9_yJGYOSo