After millions of years of evolution, human males will adapt to become nothing more than a cock and balls. At this point the male's sole purpose would be to be used as a dildo for the female. The penis would be fully developed when it comes out of the womb. Much like moths, males would have a large store of fat so they could last months without eating. They would have a tiny hole on the top to act as a breathing tube. No mouth, eyes, body, or brain would be needed.
After even more time, the cock will learn to fuse with the female. The male will supply semen to the female's womb. Eventually the penis will evolve to be less self sufficient, feeding off of the blood from the female. The woman will take on many penises so that she can select cum from many mates at all times. The penises will develop uteruses themselves, and the female will adapt to lay her eggs in the penises' uteri. Allowing for the woman to incubate many children at once. In the end stage of our evolution, the males will learn to detach from the female after they are impregnated and become solitary incubation chambers for their children.