He deserved it. Gorrammit, Christo deserved that. I've never faulted Christo for being a ladies man and it's not my place to judge him. He knew what he did was wrong. So did Melanie. The moment it happened, I KNEW that Andy was going to find out and this would be his reaction.
1) If Peter had thrown the punch, Christo would have been laid out flat, not just staggered. Peter's got more, longer-simmering, anger at Christo to unleash.
2) Christo is completely self-centered, it's all fun and games to him. None of the backlash of Christo's other transgressions (like going for Peter's boss's daughter) has ever landed back on him....until now. Time to grow up a little, son.
I don't see anyone calling out Melanie. She's every bit as guilty as Christo. It really doesn't help that Andy's been cheated on before, so if he isn't as over it as he's said in the past, this could lead to a serious blow-up
Answering the question of when was this: Melanie's birthday, girls night out. Kara tries to hit on Michelle, gets rejected, gets drunk and starts kissing strangers, Lauren takes her home. Michelle leaves too because Christo arrived, so Melanie is left alone and still wanting to celebrate. Melanie and Christo go out for pizza and go to her apartment to play videogames, and end up having sex.
Christo's vs. Melanie's fault
In reply to everyone blaming Christo over Melanie:
Now, this is obviously just a theory since I don't know what people's true feelings are on the matter, but I THINK the reason everyone is calling out Christo over Melanie is because this is his M.O. He sleeps around a lot, and while no that is not grounds for putting blame mostly on him, he does it with very little regard for anyone's feelings, but at the same time he has no problems calling out someone else for not regarding his (i.e. Michelle and Peter).
As for Melanie, this is not something she is known for, and her motivations for sleeping with Christo (I believe) stemmed from her feeling neglected by Andy because his obsession with becoming a rockstar took precedence over their relationship. That said, that doesn't mean that I condone her actions, nor that she doesn't deserve just as much responsibility as Christo. They both made a choice in that moment. They both betrayed Andy.
Because of those reasons, I can't say for everyone, but my feelings towards both parties are different. I'm pissed off at Christo for only ever thinking of himself and hurting his friends in the process, but I'm disappointed in Melanie for not being better for Andy, AND sleeping with CHRISTO of all people (I'm sorry, but I've not been a big Christo fan of late). It was a big step for Melanie to put so much trust in Andy because of his former reputation and the abusive relationship she just left, but it was an equally big step for Andy to trust Melanie after his past with relationships and being betrayed before.
We're just seeing the fallout with Andy's and Christo's relationship... Melanie's will be next, and I don't think it's going to be pretty either. I don't see how they come back from this.
It's in one of the (canonly scripted but guest artists) storylines, I think it's one where readers ask the characters questions. This was confirmed in the storyline where Patrick gets pissed Andy is dating his sister, and he reminds him that he cheated on Kara (inside a closet or something?)
If I'm not mistaken (which I could be) I think both Kara and Andy slept around on each other. But you are right, Andy has even admitted that he was not the "great boyfriend" that Patrick tried to make him out to be when Patrick was trying to discourage Peter from getting involved with Kara.
That being said, if I remember correctly, Andy has remained faithful to Melanie since they've been together. But it's been so long since then, I don't remember what One-Inch Punch got up to when they were on tour so Andy could have slept with some groupies, but I don't recall that happening.
I've been re-reading the comic from the beginning because I was trying to remember some of the foreshadowing Peter mentioned in Book One just to see how much of the recent events might be relevant. And to refresh my memory.
"If I'm not mistaken..." UPDATE
Ok. I finally got to the "Ask Pictures of You" section where the readers ask the characters questions. I was wrong with Kara also cheating on Andy. It was just Andy.
But still, Andy took a big step in making a commitment with Melanie where he has never done that with anyone before.
Though really, this is all unimportant, and me being wrong about the events of Kara's and Andy's relationship/breakup has nothing to do with my initial theory behind ppl calling out Christo vs. Melanie. But thank you for bringing it to my attention. At least I've been "re-educated".