And this is these two not communicating that there's more going on here than work/fuck buddies: It wouldn’t matter to Arthur just what Merlin meant by it, but this goodbye wasn't his to understand. This is goodbye and Arthur is taking it as another bout of awesome sex and expecting more later.

Arthur's admission that they need to have sex all the time implied that he wanted this to continue beyond the job but Merlin had already made up his mind that Arthur wanted this to be casual and for Merlin it was anything but.

Arthur, of course, had convinced himself that casual was all he could do. Idiots. Now he gets to bask in the not glory that is an anonymous hotel room with an equally anonymous bodyguard. It wasn't getting attached that was the mistake, it was not admitting how attached you are, Arthur.