Manga depicting age gap/underage relationships get quite a bit of flak here on DS as well. A somewhat recent example being Yuzumori-san, where the forum thread was largely people loudly shouting at each other concerning the subject for the first good few pages.
Whether it's age gap or other things depicting real life taboos, a large amount of people always try to look at things as black or white, with nothing in-between, justifying or damning characters and how they behave in a polarising way, rather than in greyscale (e.g. see Mousou Envy's forum thread).
When it comes to fiction, such discussions are quite unnecessary at best (after all, there are no real victims or consequences), however even real life has its share of grey zones, with things that many people would be hard-pressed to condone. For example, I know someone who has fond memories of consensually fooling around sexually with her sister, while on the other hand I also know someone who's a victim of systematic, incenstual rape, and suffers from PTSD as a result.
Knee-jerk reactions end to focus on examples like the latter, and completely dismiss the very existence of examples like the former when it comes to sensitive and/or taboo topics.