That may not be entirely true based on a little research:
How do I tell the sex of my frog or toad?Sexing frogs depends on the species. There are a number of tropical species that are dimorphic in regards to color, with the males being more colorful than females. In Minnesota, Bullfrogs and Green Frog males have larger eardrums than females. In some species males will have enlarged thumbs. Most good field guides will provide descriptions of the differences.
Under certain conditions, some frogs can change their sex!
Check it out So, He may actually be a She