I think I have a different definition of the word "asshole" than you do. To me, the word implies malicious intent and/or carelessly hurting someone for no reason. I didn't get the impression that Marten did what he did to be malicious toward Padma or that he didn't give any thought about her feelings, more that he felt that she was leading him on and hurting him and he was trying to stop her from hurting him further. Obviously, he didn't handle it as well as he could have, but the "asshole" label doesn't really apply unless you have a different definition for the word.
From Padma's perspective:
She knows the break-up is coming and it will suck. She doesn't know if Marten can make it better (this is supported by the remark in bed the last time they met). She might want him to prove to her that he can make her feel better. To come over without her asking. To prove he is as emotionally invested as she is. To make something better. But he doesn't. She does not see that she is being just as childish as he is. She just wants it to suck less. She wants to see that she's not just a stupid girl falling in love with the wrong guy. Who knows? Maybe she just panicked.
As the time winds down, she understands that he isn't going to come. She decides, against her earlier judgement, that she likes him enough to see him one last time. Despite not immediately agreeing with her that they should have been together longer, she thinks he deserves it. That they could have one romantic night, and leave on a good note. And then he blows her off, without giving a reason less nebulous that "I'm busy". Leaving her alone, not sure if he really cares, not sure if her worst fears are true or not. In that last act, he didn't behave like a boyfriend. He acted like a self-centered asshole.
This is just one possible scenario. It's doing the exact same thing as Marten did only earlier, because he triggered it with a stupid remark he didn't think about. Did Marten consider this? Not according to his conversations with the rest of the cast. He doesn't even understand his own reaction. It's a knee-jerk reply, which he can't rationally explain. So according to your definition, he carelessly hurt Padma for no reason. And is an asshole.
No one is saying Marten is a scumbag, looking to hurt Padma (well, a few are). Everyone should know he's too passive to do that. He felt unwanted when she didn't want to be with him, and that sucks. Thinking about her hurt him, so after a week of getting pushed away and trying to not think about her, he just acted on instinct. But he obviously didn't consider the fact that Padma could be just as hurt as he was.
That being said, it is kind of about time. He's been trying to please everyone since the comic began, and this is the first time he didn't (even when he broke up with Dora, he did it because she left him no option to make things better). He did it clumsily, and mishandled it terribly. But it's a step on the right path. He did something (without understanding why) because HE WANTED TO DO IT. Good for him. People shouldn't be nice all the time. That world would be hypocritical and disgusting.
I'm still wondering why Faye's blatant disregard for Marten's feelings when he's been on edge for so long isn't getting more attention though.