“On three separate occasions, a research team led by William A. Haddad and de Bruyn spotted young male seals sexually coercing what appeared to be healthy penguins of unknown gender.
…
‘This really made us sit up and take notice,’ said de Bruyn.” (http://www.bbc.co.uk/earth/story/20141117-why-seals-have-sex-with-penguins)
Being mounted by a seal would do that.