this is Gen 5 remake of the first series. This series displays the visible physical gender differences between Pokémon. They taped on a piece of paper to make Pikachu’s tail look like the female shape in that one crossdressing episode. There was also a female Girafarig which had the correct color pattern, at one point in the previous series. It’s deliberate. Animators pay attention to it.
Reminder that there is a gender difference between male and female Butterfree.
This is a male Butterfree from the back in the most recent games.
And this is a female. Note the dark spots on the bottom wings.