The three-year-old darling of Berlin Zoo was given a female companion, Giovanna, last year but the German chapter of People for the Ethical Treatment of Animals (PETA) warned against their mating.
The group's zoo expert, Frank Albrecht, noted that Knut and Giovanna, known as Gianna for short, had the same grandfather.
Any offspring would threaten the genetic diversity of the polar bear population in Germany and risk susceptibility to a condition known as "incest depression", he said.
"Knut fans need to know that only Knut's castration would allow a long life together with Giovanna," Albrecht said in a statement.
Gianna had lived in Munich but was placed with the strapping Knut in Berlin due to construction work on her own den.
After chilly introductions, the two have gradually grown quite close. The housing arrangement, however, was initially intended to be temporary.
Knut drew an outpouring of sympathy around the globe after his mother cast him out as a cuddly cub.
The cult of Knut spread around the world and his first public appearance attracted some 500 reporters, including around 100 camera crews.
At the height of his fame, Knut featured on the front page of a host of glossy magazines, including Vanity Fair.