Fourths and fifths come in only one standard size called 'perfect', not major or minor

Unlike seconds, thirds, sixths, and sevenths (which each come in major and minor sizes), fourths and fifths have a single standard size called ‘perfect.’ A perfect fourth spans 5 semitones and appears whenever you move from any letter class to the fourth letter class above it — almost universally. A perfect fifth spans 7 semitones similarly. The exception is the pair F-B (4th) and B-F (5th), which are each a semitone off from perfect and become augmented fourth and diminished fifth respectively — both of which equal the tritone. The word ‘perfect’ marks these intervals as the historically most stable consonances.

Examples

C up to F = perfect fourth (5 semitones). C up to G = perfect fifth (7 semitones). F up to B = augmented fourth (6 semitones = tritone, not a perfect fourth).

Assessment

Why is there no ‘major fifth’ or ‘minor fourth’? Identify the one pair of natural letter classes that does not produce a perfect fourth, and name the interval that results instead.