At what point does a character with a penis and no significant breast tissue stop being a girly male and become a dickgirl?
The most simplistic reading of gender tagging guidelines would say that a "flat-chested dickgirl" would just be tagged as a male, with a likely addition of the girly tag, here's about seventy images (and possibly more) where apparently that interpretation is invalid.

The link gives a 400 Bad request message

D4rk said:
The link gives a 400 Bad request message

It's link to this tag search: flat-chested dickgirl -breasts -girly

I really hate talking about gender related taggings and usually take back seat, but do remember that genitalia and breasts aren't the only characteristics that can determine gender of a character. Especially young or feral characters can easily have flat chest but still look entirely female instead of girly male if the dick was removed.

As for gender tagging guidelines, remember that those are still just guidelines. Even if those are usually reflected onto, that doesn't mean that they are set in stone or 100% objective. That means there will be cases like flat-chested dickgirls for certain.