“Sons of men” in the bible is written in a different language. In most cases, what is meant is children, or at least offspring, and in that sense could refer to both genders. In Psalm 33, the construction is probably used to emphasize that they are merely sons of men, as opposed to a son of God. But I wouldn’t defend that point too vigorously.
In St. Leo’s case, he is not meaning to reference offspring (and Latin did have better gender-neutral terms for offspring), but appears to use “sons of men” because it is simply more gender-specific than simply than “men.”