A few answers here: http://www.reddit.com/r/AskHistorians/comments/2aomqf/how_did_polygamous_cultures_have_enough_women_to/

To sum up, though:

• Polygamy wasn't universal in cultures that practiced it (mostly concentrated at the top of the social hierarchy), so the imbalance wasn't necessarily that extreme/destabilizing.^1

• Men in those cultures may have had higher mortality rates than women, especially if they went to war a lot.

• If the population is growing rapidly, and men are expected to marry later than women, then you could have more "marriage-age" women than men, as long as the growth trend continues.

^1 This is not true of modern polygamist groups like the FLDS, whose practice of polygamy does leave a lot of young men without opportunities for marriage. Their response has been to find reasons to exile them from the community.