The Mister Seahorse trope in general. The male body is not equipped or designed for the production, incubation, or definitely the birthing of babies. The lack of a womb also means that a male incubating a baby would literally have a parasite growing in-between vital internal organs, possibly damaging them, possibly cutting off something important, and almost certainly killing either him or it. He might live if the infant gets killed, as without the protections of a womb the fetus would likely be immensely more susceptible to damage from white blood cells, physical trauma, the man bending over, etc, and the chances of the fetus not making it are just as good as the fetus killing the man, which is to say, high. If, by some miracle, both man and fetus survive nine months, any concievable route of exit is even more unimaginably painful than for females. The vagina is designed to stretch greatly to accommodate infant exit, whereas the anus and penis, the only two possible exits, are most certainly not designed to stretch much, if at all, and would as such be stressed beyond their ability to cope if an infant were to attempt passage. A male giving birth, lacking a vagina, will inevitably result in the stretching of one of those two regions. A human male doesn't have the necessary equipment to go into labor in the first place: no uterus, hence no uterine walls to do the pushing. Were a man ever to be successfully implanted with an embryo in his abdomen, he'd have to have a cesarean section like in the movie Junior.