First of all, when the 172 question came up i would of countered with "but it does have a dihedral, albeit less than a low-wing" (assuming you accurately phrased the question u got), then added that while, as you correctly stated, highwing aircraft are more stable, the 172 also has struts that produce drag, which make up for some of the stability thus requiring a bit less dihedral.
The BAE 146 has an anhedral because a highwing & sweptwing combo produces an aircraft that is too stable. Oh and of course because it looks better...
