But not really that radical. After all, Nemo is a clownfish. And to quote Wikipedia:
"In a group of anemonefish, there is a strict dominance hierarchy. The largest and most aggressive female is found at the top. Only two anemonefish, a male and a female, in a group reproduce through external fertilization. Anemonefish are sequential hermaphrodites, meaning that they develop into males first, and when they mature, they become females. If the female anemonefish is removed from the group, such as by death, one of the largest and most dominant males will become a female. The remaining males will move up a rank in the hierarchy."Nemo's "Dad" may well have been a male when he fathered Nemo. But if he was top clownfish, when his mate died, he went on to be a female.
So Nemo's Dad underwent a transition to female. Two lesbian humans? Finding Dory isn't really trying.