I don't get it. Why couldn't Cro Magnum and Neanderthal interbreed? What would prevent them from doing so? They sure looked enough like humans.
It's clear that evolutionary divergence is not a linear temporal curve. Some (sub)species diverge rapidly while others diverge much more gradually. Based on genetic tests of modern humans and paleolithic fossils, the most recent common ancestor between Neanderthal and Cro-Magnon was extant about 500,000 years ago. There are many possible reasons that may have prevented them from interbreeding once the two lineages were reunited. It's entirely unclear which, if any, was the culprit, but the stubbornly persistent inability to find evidence of admixture (short of very creative interpretation of some evidentiary fragments) strongly suggest that one or more of these factors was the case. We may never know which.
It's worth noting though that recent genetic research indicates that the genes for language and intelligence evolved rapidly in the time frame that modern human ancestors were separated from Neanderthals, so that supports the notion of a very rapid divergence.
For that matter, I'd say it's an open question whether Homo sapiens could interbreed with Homo erectus, if the two were somehow united in coitus. It's even possible that a modern human could interbreed with a Homo erectus but not with a Neanderthal (it's less likely, to be sure, but it's possible). It all depends on where the genetic divergence took place in the respective populations.
My guess is that if it happened it was of such a low incidence rate as to be a freak occurrence of no meaningful consequence. Kind of like Caligula marrying his horse. Sure, it proves that out of billions upon billions of humans that have ever lived, at least one married a horse (if the slur is true). Big whoop. It hardly establishes any kind of norm.