He picks two or three that his instincts say will provide the best genetic material to provide good quality heir, he clubs her over the head and proceed to spread his seeds. He cannot pick too many females or his herd may starve from the lack of resources to hunt food.
All of this will have to take place in quick time, because at 16 years of age, a young caveman probably have a further fifteen years of seed production before his seeds run dry or being ingested by a big-toothed-big-cat (no, not Fat Cat).
Hypothetical Modern Days
A typical modern male will scour the Internet and find thousands of potential mates.
Due to society and religious conditioning, he is only allowed to choose one potential female to procreate with.
Due to media's perpetuation of the 'Love & Romance' myth, he must spend a few months wooing just one at a time, taking her out to dinner and parties and weekend getaways, all the while expending his resources to do so.
Before too long he find this potential mate is a psychotic/neurotic/controlling/dependent nut-job. He must spend a further few months trying to wash his hands of the relationship and reset his life to a point where he is ready to find another mate.
Reality is, he is not Rupert Murdoch and he is unlikely to find anyone that he feels is the pinnacle woman, who is worthy of carrying his heir, i.e. "The One™".
He will die, a single lonely man, from ingesting too much bovine burgers and alcoholic beverages.
Inspired by Sam de Brito.