"Event took place on a beach on Marion Island, a sub-Antarctic island that is home to both fur seals and king penguins."
"At first glimpse, we thought the seal was killing the penguin"

"The 100kg seal first subdued the 15kg penguin by lying on it.
The penguin flapped its flippers and attempted to stand and escape - but to no avail.
The seal then alternated between resting on the penguin, and thrusting its pelvis, trying to insert itself, unsuccessfully.
After 45 minutes the seal gave up, swam into the water and then completely ignored the bird it had just assaulted, the scientists report."

"Why the seal attempted to have sex with the penguin is unclear. But the scientists who photographed the event speculate that it was the behaviour of a frustrated, sexually inexperienced young male seal."

"this is thought to be the first recorded example of a mammal trying to have sex with a member of another class of vertebrate, such as a bird, fish, reptile, or amphibian."

http://news.bbc.co.uk/2/hi/science/nature/7379554.stm