phonemes
Explanation
The Joke
The comic depicts a couple in bed (shown as white text on black panels to indicate darkness). One partner asks the other to "rare phonemes" while they make love. The other partner begins producing various phonemic sounds -- "ep! gp!", "do! oooo!", "thhh!" -- which are enthusiastically received with cries of "more! more!" and "I'm so close!" until climax.
After the encounter, one partner says "a voiceless dental fricative? Get off me." The other protests, noting that it occurs in very few languages, but is told "you said 'rare phonemes' -- a theta is rare among languages, not among phonemes." In other words, the sound /th/ appears in relatively few of the world's languages, but it is common within languages that do have it (like English and Spanish). The partner who made the error tries to recover, then announces they will be in the bathroom. A final exchange asks if they are "making click consonants in there," to which they reply, "I am finishing the job."
The Humor
The comic takes the concept of a bedroom role-play scenario and substitutes linguistics jargon for the typical content, creating an absurd juxtaposition between sexual intimacy and academic pedantry. The humor escalates when one partner is rejected not for any typical bedroom faux pas, but for producing a phoneme that is technically "not rare enough" by the wrong metric -- a distinction so pedantic that it could only matter to a linguist. The final gag about click consonants in the bathroom adds another layer, implying that the rejected partner is continuing alone with the most exotic phonemes they can produce (clicks, found primarily in certain African languages like Xhosa and Zulu).
References
Phonemes are the distinct units of sound in a language that distinguish one word from another. The voiceless dental fricative (the "th" sound in English "think") is represented by the Greek letter theta in the International Phonetic Alphabet. It is indeed relatively rare across the world's languages -- only about 7-8% of languages have it -- but is extremely common within English. Click consonants, found primarily in the Khoisan and some Bantu languages of southern Africa, are among the rarest sound types in human language globally.