Some covers of 'Dejah Thoris - Warlord of Mars'
are more faithful to her nudity and 'perfect beauty' than even Frazetta's paintings -his women are a little plump below the waist and a little skinny above.
Barsoomian biology is more... bizarre (read: incredible) than any steampunk 'weird' technology; your 'obvious dangly bitz' indeed seem to refute the Monotreme hypothesis, so convenient to explain the Red Martians' surprising lack of taboo about total nudity ('real' pubescent humans wear at least a micro loincloth or a penis shaft). Then John Carter was obviously 'different' anyway -lighter skin, and more hairy (and I doubt Red Martians have facial hair)- so maybe this... peculiarity was noticed but not commented upon, Barsoomians are accustomed to so much anatomo-morphological diversity? Besides, I should learn more about the intimate functioning of male Monotremes: are they totally deprived of external genitalia (like birds) or is the 'dangly bitz' stored inside when not in use (as for male kangaroos)? In the later case John Carter would look to Martian eyes as if suffering
priapism (so much the more with a naked 'perfectly beautiful' Dejah Thoris within view...
). It would have be very embarrassing and impolite for Martians to comment about it. And if indeed 'at ease / resting' male Martians sport no external genitalia, John Carter is too reserved to comment about his secret satisfaction: to gloat about his extraordinary jumping performance in Barsoom lower gravity flatters his ego enough
As for the ample bosom, Monotremes *do* produce milk, and according to Desmond Morris the obtrusive breast of female human is a consequence of the upright stance / bipedal walk, so...
Anyway I love to imagine John Carter's face when some sensitive part of his anatomy met for the first time Dejah Thoris' eggshell laying apparatus