Your alleging that he has forgotten "what casual sex usually involves" (citation needed, by the way) implies that he has been adhering at some point to your definition of casual sex, which apparently involves systematically kicking your temporary partner out of your life forever.
So, apparently, your definition of casual sex means treating people like throwaway commodities.
Sorry to but in, BUT - Do you think Delilah treated Marten like a throwaway commodity? I think you're looking at the situation with a bit of an extreme point of view.
I thought she was pretty straightforward about the whole thing, she didn't just discard him. She apologized for waking him up, called him cutie, stressed how much fun she had but that ultimately she didn't want to make it into something more. Which is fair enough. She really doesn't owe him anything, does she? Just because they had sex (which Marten initiated with his makeouts flirtation, btw) doesn't mean she has to be a part of his life from that moment on, does it?
Sure, the name forgetting was unfortunate, but it was a wild night and clearly Marten is guilty of the same thing as proven by Cosette.
I sort of agree with greywolfe in that Marten, when you really get down to it, doesn't actually have anything to "complain" or be upset about. He made a pass at a girl he didn't really know. It worked and they had sex and while it was fun she wanted to leave it at that. Oh, and they both forgot each others names. They both had the same amount of input in the way the story turned out, no one was used, everything was made clear at the appropriate time so... what's the big deal?