A girl and a boy hugging on a bridge
Cat and dog fighting on a boat.
He refers to a boy and girl not a cat and dog. They cannot be hugging and fighting simultaneously. They cannot be on a bride and on a boat simultaneously.
The first sentence is about a girl and a boy while the second sentence is about a cat and a dog.
A boy and a girl are different from a cat and a dog and people cannot be hugging and fighting at the same time.