Given they didn’t leave written records, we don’t really know.
There was a National Geographic article on the Hadza, a tribe of hunter-gatherers in Kenya, who didn’t have any institution of marriage, and they just went from relationship to relationship. But interestingly, there was an old couple who had been together all their life.
Also a major underpinning of marriage is an understanding of physical paternity. It seems probable that this was not properly understood in the Stone Age. There is evidence of it not being properly understood even into modern times. For example, C.P. Mountford, in ‘Brown Men and Red Sand’ tells of an Aboriginal tribe in central Australia in 1948 who didn’t understand the connection between men, sex and babies.
Assuming this ignorance was the original condition, and was more widespread in the Stone Age, then there would have been less reason for marriage.