Three points here, the main one (the one that the cartoon turns on) being the ambiguity of romance (in Romance languages vs. romance movies). Then there’s the question of counting Romance languages. And, finally, the side issue of the dismissive exclamation my butt!.
(Hat tip to Jack Hamilton.)
1. Ambiguity. OED3 (Sept. 2010) has an extraordinarily long etymological note on romance (noun and adjective), and a long, tangled set of sub-entries and citations. The story starts in French, where the historical antecedents refer to (in rough chronological order): the vernacular French language (from the language of the Romans in France); Vulgar Latin; a French text that’s a translation of a Latin original; a medieval narrative written in the vernacular rather than Latin (especially relating extraordinary adventures of some hero of chivalry).
Then, in English (and also in French): characteristic of the style or content of such literary works; a narrative of sensational or exciting events; extravagant fabrication; the quality of appealing to the imagination; ardor, especially of warmth or feeling in a love affair; love, especially of an idealized or sentimental kind (first cite not until 1858); a love affair (from 1844 on).
Meanwhile, in a separate development: the vernacular language of medieval France; (later) any of the related Romance languages (the vernacular languages descended from Vulgar Latin, some later standardized). Romance language(s) appears in written records from 1421 on.
So we get the teeanage boys confounding the two major etymological threads, and hoping, goofily, that the Romance languages are the languages of love.
2. How many Romance languages? The boys look to Wikipedia for information on Romance languages, and in fact the entry is a generally good source. It is especially reasonable on the vexed question of how to count the Romance languages:
Because of the extreme difficulty and varying methodology of distinguishing among language, variety, and dialect, it is impossible to count the number of Romance languages now in existence, but a restrictive, arbitrary account can place the total at approximately 25. In fact, the number is much larger, and many more existed previously. Nowadays the six most widely spoken standardized Romance languages are Castilian Spanish (about 500 million), Portuguese (about 240 million), French (about 250 million), Italian (about 70 million), Romanian (about 30 million), and Catalan (about 14 million). Among numerous other Romance languages are Corsican, Emiliano-Romagnolo, Galician, Gascon, Lombard, Mirandese, Occitan, Piedmontese, Aromanian, Sardinian, Sicilian, Venetian, Asturian, Neapolitan and Friulian. [There are links to entries for all of these language forms.]
In the cartoon, Morton runs through the six standardized languages listed in the Wikipedia: in order, French, Castilian Spanish, Italian, Portuguese, Romanian, and Catalan.
3. My butt. Here the story begins with a piece of slang, the dismissive exclamation my arse!, which the OED (draft addition of Dec. 2002) labels as chiefly British and Irish English, with cites from 1933 on.
The OED2 entry for the American English ass as the correspondent to British arse (which it has from 1860 on) lacks the American exclamatory my ass!, but you can google up plenty of American instances. (Presumably this will be added when the entry gets revised.)
Exclamatory my butt! is hard to find, however, and its appearance in the Foxtrot cartoon looks like a toning down of my ass! (replacing the mildly taboo ass by the merely vernacular butt) — not a very likely event in talk between two teenage boys, but not entirely surprising in a cartoon with a broad readership (the strip appears in over a thousand papers).