All clownfish are born male. When clownfish unite in a group, the more dominate male fish (the leader of the group) will turn female. The female is like the leader of the group. If the female dies, another clownfish of that group will turn female and take place of the group leader. Groups are made up of all males and one female. In finding Nemo: [spoiler]The father and mother have eggs, the eggs get eaten except for one and the mother dies.[/spoiler] So currently, the father is in a group by himself since there seems to be NO other clownfish in that part of the ocean. So the Father has already turned female. Then Nemo (male) is born. Everything has instincts to mate. So... Eventually Nemo is going to have sex with his father unless other clownfish are around in the movie which appears not