Jealousy
Jealousy
[Dark scene shown, with Cueball and Megan sitting in the moonlight let in by the only window.] Megan and I first met at a party at her sister's. We hit it off, opened up, shared secrets, and talked about everything. Around us, the party waned, but we hid from sleep together, talking through the deepest hours of the night. The dawn found us curled up on a couch, asleep but still together. That experience, connecting with a stranger and falling recklessly in love is one of life's greatest joys. And now that you're married, you'll never experience it again. It's the price you pay for everlasting love. It's a small one, but I hope it stings a little. Anyway, I wish you and Megan the best. ...Hey, man, you asked me to do a toast.

In the first four stanzas of this soliloquy, Cueball reminisces fondly about his meeting of Megan at a party where they quickly had a romantic rapport and spent the night together. The next two stanzas are a lamentation that Megan is now married and can never have that same experience again.
The final stanza reveals that this is not merely Cueball waxing poetic about a previous love, but that he is actually making a toast as the best man at Megan's wedding reception - certainly an occasion when it is wildly inappropriate to discuss the previous romantic involvements of the bride. In modern times, it is not unexpected that the bride at a wedding will have had previous relationships with someone other than the groom, but a wedding is no time to mention them.
The title text implies that the groom had no prior knowledge of Cueball's and Megan's dalliance, and therefore the toast is even more inappropriate - going beyond discussing the previous romantic history of the bride, to actually revealing previously unknown such history to the groom.
Presumably, Cueball's having been selected to be best man indicates that he is a good friend of the groom (as the groom normally selects the best man). The title hints that Cueball is envious of the groom, since his previous encounter with Megan clearly did not result in a lasting relationship, while the groom's clearly did. This suggests that he may be intentionally (perhaps drunkenly) making an inappropriate toast to embarrass the couple as a result of his bitterness.
This was only the third time the name Megan was used in xkcd, the first time being in 159: Boombox .
[Dark scene shown, with Cueball and Megan sitting in the moonlight let in by the only window.] Megan and I first met at a party at her sister's. We hit it off, opened up, shared secrets, and talked about everything. Around us, the party waned, but we hid from sleep together, talking through the deepest hours of the night. The dawn found us curled up on a couch, asleep but still together. That experience, connecting with a stranger and falling recklessly in love is one of life's greatest joys. And now that you're married, you'll never experience it again. It's the price you pay for everlasting love. It's a small one, but I hope it stings a little. Anyway, I wish you and Megan the best. ...Hey, man, you asked me to do a toast.
