Homo Sapiens survived quite a long time without much more then an open fire for a heat source, including periods where the average temperature was much lower then it is now.

In fact, barring major disease and wide-spread war, there has not been a period in history, that I can remember from my grade school lessons, that the population has not been on a steady rise.

Don't worry overly much about our ability to procreate in cold weather, we have done it before, and as long as we have the fertility and a large enough pool of genes, life will find a way.