Venus and the Seven Sexes

It is written in the Book of Sevens:
When Plookh meets Plookh, they discuss sex. A convention is held, a coordinator selected, and, amid cheers and rejoicing, they enter the wholesome state of matrimony. The square of seven is forty-nine.
This, my dear children—my own meager, variable brood—was the notation I extracted after receiving word from the nzred nzredd that the first humans to encounter us on Venus had at last remembered their promise to our ancestors and sent a cultural emissary to guide us on the difficult path to civilization.
Let the remaining barbarians among us cavil at the choice of this quotation; let them say it represents the Golden Age of Plookhdom; let them sneer that it shows how far we are fallen since the introduction of The Old Switcheroo by the gifted Hogan Shlestertrap of Hollywood California U.S.A. Earth.
The memory of Hogan Shlestertrap lives on while they disappear. Unfortunately—ah, well.
Please recall, when you go forth into the world to coordinate your own families, that at this point I had no idea of the kind of help the Earthman wanted. I suspected I had been honored because of my interest in literary numerals and because it was my ancestor—and yours, my dear children, your ancestor, too!—the nzred fanobrel, to whom those first Earthmen on Venus had made the wonderful promise of cultural aid.
A tkan it was, a tkan of my own family, who flew to bring me the message of the nzred nzredd. I was in hiding at the time—this was the Season of Wind-Driven Rains and the great spotted snakes had come south for their annual Plookh feed; only a swift-flying tkan could have found me in the high grasses of the marsh where we nzredd hide at this season.
The tkan gave me the message in a few moments. It was possible to do this, because we had not yet been civilized and were still using our ancestral language instead of the cultivated English.
“Last night, a flame ship landed on the tenth highest mountain,” the tkan told me. “It contained the long-promised emissary from Earth: a Hogan of the Shlestertrapp.”