The problem was that the female Humboldt penguins have proven too shy in their advances, the director of the zoo in the northern port city of Bremerhaven said.
“The Swedes will not make the first move,” Heike Kueck said.
The females were flown in last year in a bid to bring the males to mate and help save the Humboldt species from extinction.
Kueck said last year she was optimistic the initiative would be successful because zoo keepers had noticed that at one point a female penguin had managed to cause a couple of males to “separate”.
The zoo has 10 male penguins of which six have shown strong signs of preferring male company and formed couples among themselves.
The initiative to “turn” the penguins and make them mate had prompted a furious response from gay rights groups.
In a statement posted on its Internet website, the zoo on Wednesday sought to defend itself from fresh criticism.
“We will be delighted if the penguins form even one heterosexual couple and manage to produce first an egg, and then a little one,” it said.
“But of course we accept the male couples that have formed and we are not trying to enforce heterosexuality, as we were accused of doing last year.”