The penis of an Eastern Grey Kangaroo, out of the sheath but not fully erect. Most marsupials have a bifurcated penis, but as you will note from these photos, both the grey and red kangaroos only have a single tip.
The debris on the end of the penis is common among kangaroos.1
The scrotum of the Eastern Grey Kangaroo. In common with all marsupials, the scrotum hangs in front of the penis.2 The male has a fine degree of control over how far to extend or retract the sac; fully retracted when preparing to fight (to protect these sensitive organs) or hanging loose (to insure the testicles remain cooler than the rest of the body).
Erect penis of a Whiptail Wallaby (Macropus parryi).
Erect penis of a Swamp Wallaby (Wallabia bicolor).
1 Johnston, S. D., Blyde, D., Gamble, J., Higgins, D., Field, H. and Cooper, J. 1997, 'Collection and short-term preservation of semen from free-ranging eastern grey kangaroos (Macropus giganteus: Macropodidae)', Australian Veterinary Journal, vol. 75, no. 9, pp. 648-658.
2 Tim Flannery had some interesting comments on this unusual aspect of marsupial anatomy in his book Country (pp. 60-61, bibliographic details available under the FAQ):
All marsupials share this strange anatomy, and if you have the necessary equipment and are so inclined, palpation of your own wedding tackle may convince you that your ancestors possessed a similarly inconvenient arrangement. To prove that the reverse order is the primitive condition for mammals, trace your penis posteriorly until you find its base, very near the anus (a more specialised offshoot of the cloaca). The ensheathing of the organ in skin and tendons so that it exits the body well forward of this point has been the work of untold millions of years of evolution.
The original, forward position of the testes in our ancestors can also be demonstrated by our anatomy. Gently pinch your abdominal wall just to the sides and in front of your penis's point of entry into the world. You should be able to feel the cords from which the testes are suspended. Before you were born your testes migrated through your abdominal wall then took a downward turn to enter the scrotum, there to reside in a sensible placental manner behind the business end of the penis. But in our distant ancestors, which lived before this migration occurred, they clung to the stomach wall below the belly button. This is pretty much what you see in kangaroos, except that they have evolved a capacious scrotum to house them.