Where does Chesterton mention that popular culture suggests an innate understanding of sexual fidelity

In *Mere Christianity*, in the chapter on Christian Marriage, CS Lewis says, > "As Chesterton pointed out, those who are in love have a natural > inclination to bind themselves by promises. Love songs all over the > world are full of vows of eternal constancy. The Christian law [of marital fidelity] is not forcing upon the passion of love something which s foreign to that passion's own nature......" I'm looking for the place where Chesterton says this.