Any biologists in the house? Because seems to me the physical aspects of a womb, and the immune system aspects, seem to me to make it a huge difference between, say, laying eggs, and carrying a little one or two to term?
ALthough there are some fascinating skinks where some do eggs, some do live birth, and article says:
"That suggests that it is rather easy to evolve. After all, to give birth to live young, a female simply has to keep the developing offspring inside herself for longer. But this gives rise to a host of challenges: she must provide it with food so that it can keep growing, protect it from her own immune system, which is liable to attack it, and ultimately eject it in a way that doesn’t kill it."
Whereas the male reproductive system, let's face it, evolving from a cloaca to a penis is hardly the most impressive feat! Just elongate a tube and stick some erectile tissue in there, or perhaps a bone or two, not much difference! Although apparently female dinosaurs did the same calcium from yer bones trick!
Seems yet again females had to do all the heavy lifting as regards evolution of reproduction, males just obsessed about size, had fights, and created flashy displays of useless stuff................