I'm not really a fan of this reasoning. Can you explain how a 50 year old having sex with a 16 year old is more harmful than a 24 year old having sex with a 16 year old? The only reasoning I can see is "gross", which really shouldn't be the basis for laws.
I am in favor of "romeo and juliet" laws, which can be applied either when one half of a couple goes over the age of consent or when both are minors (there should definitely be room for leniency in these cases). I'm not in favor of treating adults differently based on their age though. If a 50 year old shouldn't be having sex with a 16 year old, then a 24 year old shouldn't be either.