I assume you are talking about our Hunter-gatherer ancestors based on the timeframe, but I'm not sure what you mean by serious consequences. Could you expand on that a bit?
I suspect monogamy as we know it is a response to the invention of agriculture, and we have closer relatives (the bonobos) who have sex much more freely than some of our other closer relatives.
EDIT: Whoops I misread the abstract of the article, that doesn’t really give evidence towards my claim. However STDs did exist and I’m assuming people knew about them. Pregnancy itself would be a deterrent
I suspect monogamy as we know it is a response to the invention of agriculture, and we have closer relatives (the bonobos) who have sex much more freely than some of our other closer relatives.