Yes, it wouldn't make sense if stone age people just hung around in couples. Haven't you watched the Flintstones?

As far as cock size goes, humans are one of the few animals to have sex purely for pleasure. It helps the couple to bond more closely, and of course the more pleasurable the sex the closer the bonding, one would assume. Maybe that's why larger cocks WERE selected for - human cocks are very large relative to other apes. Tell that to a monkey next time he insults your mother.