There is not a 100% overlap between a character having an androgynous appearance and being unable to determine their sex. A character may, for example, be seen nude or in a sexual situation, making their sex obvious, yet still have an overall androgynous appearance. Tagging such an image with both their sex and the contradictory ambiguous_gender doesn't exactly make sense, and currently the only other option is just not tagging one.
Updated by Marbles