Title text: Well, sex is like a velociraptor: despite your movie-fueled lifelong neurotic obsession, unlikely to be found in your house.
Megan describes her relationship to Cueball with the simile "our love is like a turtle," a comparison often made when referring to a shy and slowly developing yet steady sort of romance. However, Cueball thinks that the Brontosaurus is a better comparison. His explanation refers to the fact that remains of a certain apatosaurine were initially named Brontosaurus excelsus by the paleontologist O.C. Marsh in 1879. This species was later determined in 1903 to be in the same genus as Apatosaurus ajax, which Marsh had named two years before B. excelsus: the older genus name is preferred according to convention (making the preferred binomial Apatosaurus excelsus). The term Brontosaurus therefore became a scientific redundancy (a so-called junior synonym), and had this status at the time of this comic's release. Due to the correct skull for an apatosaurine not being confirmed until 1978, the term "brontosaurus" had in the meantime become popularly associated with an apatosaurine depicted with a speculative Camarasaurus-like head, hence the "mistaken combination" mentioned in the comic.
Applied to the scenario in the comic, Cueball apparently considers the relationship without any emotional foundation and only continues it out of nostalgic motives. This conclusion counteracts the initial romantic tone adopted by the turtle simile, as comparing a romance with a falsely classified fossil is one of the least charming statements imaginable. 
The title text aims at Randall's well-known enthusiasm for Velociraptors. Megan retorts by comparing any future sex between the two of them to be as likely as finding a Velociraptor in his house. This is a common trope in modern relationships where the assumption is that it is the man who wants sex, and the woman will withhold it as a form of punishment, usually in response to her emotional needs (which, stereotypically, takes the same importance to the woman as sex does for the man) being denied by the man (in this case, she initiated a romantic conversation, hoping for him to reciprocate, and yet he took the opportunity to do the opposite). The insult has a second barb: painting Cueball as being obsessed with movies involving Velociraptors. It may also hint at his having a collection of pornography, but not necessarily anything that caters to both obsessions at once.
Randall has previously mentioned the Brontosaurus name change in 460: Paleontology. The Apatosaurus also appears in 15: Just Alerting You and 650: Nowhere.
However the status of "Brontosaurus" remains under discussion, with a 2015 study of diplodocids reporting that the more gracile fossils should be classified in a separate genus. This would re-divide the apatosaurines between the Brontosaurus and Apatosaurus genera.
- [Cueball and Megan are sitting at a bench. Megan is holding a turtle.]
- Megan: Our love is like a turtle.
- [Megan sets down the turtle and turns to Cueball. They hold hands.]
- Megan: Humble and simple, enduring by virtue of perfect design.
- Cueball: Our love is like a brontosaurus.
- Cueball: Recognized as a mistaken combination long ago, lingering only out of misplaced affection for an imagined past.
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Title text might also be a retort from Megan for what he said. 126.96.36.199 06:18, 16 April 2014 (UTC)
Agreed, I think the title text is a retort from Megan (based on the "Well," start to it). 188.8.131.52 (talk) (please sign your comments with ~~~~)
I updated the title text explanation as per this discussion. -adamaustin (talk) 14:00, 29 August 2014 (UTC)
Update add surprisingly well to this comic, suggesting that breaking up turned out to be a mistake later on. Self-updating comic without any action from xkcd! 184.108.40.206 11:31, 4 October 2021 (UTC)