This is a popular definition of a species*, but you’re applying it to a class**. Humans can’t breed with other apes, but we are apes. Biological classification (fish vs whales) is based on evolutionary relationships and traits, not whether two animals can interbreed.

Cladistically, whales are not fish because “fish” isn’t a clade. Fish are an everyday label, not a clean scientific clade or rank.

* - this is a very limited definition of species
** - fish really aren’t a class, but that’s how it’s taught in grade school