Ok, I'll say a little more, I guess. Taken on its own, this is a pretty good comic. Who hasn't been / been standing next to That Guy? The pause that panels 2 and 3 provide before the punchline is good, it increases the tension (particularly because it comes in the middle of the sentence) that is then released with the punchline (which is in panel 4; more on that later). So it's better put together than the usual comic.
But that's just taking it on its own, and alas, I can't stop there. As every single one of you pointed out, there's a GIANT amount of hypocrisy in this comic, because for years, xkcd was nothing but comics that were entirely designed to show you that the author had a girlfriend, and she was cool, and they Sexed It Up with regularity. Fred's compilation is good; I also once made a list of such comics. Also, don't forget his general obsession with sex. Anyway, as the story goes, he then suddenly and mysteriously (?) stopped telling us about his girlfriend, and instead told us things like "relationships are full of sadness and sorrow" with messages like "I wish I had a robot to be my girlfriend," and "You all should kiss people more, I have come to see," and "When it comes to love, just give up." There are many, many other examples.
I'd like to say that this comic is a knowing self-reference to those earlier days, but I don't actually think it is. Randall has shown a remarkable inability to laugh at himself and his foibles, with most references being to memes he is trying to create (like the Height poster, which was full of previous xkcd jokes). I don't think he's poked fun at anything in such a way that he implied he had done anything wrong or annoying. Only that he done things awesome and great.
Lastly, a word on Post-Punchline Dialog (PPD). It's Randy's favorite tool for ruining all sorts of comics, and he uses it to great effect here. The punchline of this comic is "My GIRLFRIEND." That is where the humor is. That is what the build up is for, that is what people are supposed to recognize in themselves or their friends. Naturally, Randy keeps going, with a shitty 5th panel that doesn't even have the most important character pictured, because it is too text heavy to include him. What the hell? He is literally shifting the point to the other guy, who is not funny. "Legally required to stab you" ? What the hell is that, some kind of 9th grade legal rhetoric you picked up? And then ending with "Did I mention I'm seeing someone?" COME ON. You made the joke already! In the previous panel! And it was a good joke! Just let it be!
Now look, if you made a joke, and can then think of an additional joke that expands on the first, that's great. That's a wonderful situation to be in and can lead to a very funny result. But if your second joke is just repeating the first joke but in a more half-assed way, well, that's going to suck.
Speaking of editors, I see that the newest Penny Arcade reality show episode is up. This one is the first in a series that is basically filming Gabe and Tycho as they write a comic. It's very interesting to see, especially given my vantage point as XKCD Critic, because you can tell how important it is that they bounce ideas off each other and really work to make sure their end result is a good comic. I do hope Randall takes a look at it, and strongly considers having an editor...