i dunno about humans. but clams.... wow.
clams begin their lives as males, developing sex organs in about a year. when they get older, they may become female. now that's hot. basically they both let their stuff out into the ocean and it makes itself. they are so sexual that if they have real sex they won't be able to bare it and can die. the "stuff" meets up somewhere downstream and makes a larva that grows a foot, grabs the sand, makes a shell, and becomes a clam.
example? two clams had sex.