Very few penguin species mate for life. Most penguins remain faithful to their mates for the breeding season, then find new mates the next year. They're often more attached to a particular nest than a particular mate.
Emperor penguins have the lowest monogamy rate of all penguins. Only 15 percent of mating couples reunite after one breeding season. Yellow-eyed penguins, however, exhibit a 93 percent lifetime monogamy rate.
In 1998, researchers observed female penguins on Ross Island in the Antarctic circle apparently trading sexual favors for stones. Every female observed doing this had already picked a mate, but went to single male penguins to make the trade. The male mates didn't mind. Two Magellanic penguins from South America have mated with each other for 16 years, as of 2012.