These tokens are from ancient Rome, and they’re called spintriae. They were only manufactured for a few years (perhaps as few as 15) in the first century, and they have no real intrinsic value, being made from brass or bronze.
Oh, and they depict sexual activity, rather than the more-common profile of an emperor.
All of that leads many people to believe that they were used as “tokens” in Roman brothels — you pay the cashier, then redeem your coupon upstairs.
But not everyone agrees. Both Salon and Cecil at the Straight Dope point to an influential 2007 essay byGeoffrey Fishburn called “Is that a spintriae in your pocket, or are you just pleased to see me?” (pdf).
He makes some interesting points — there’s no evidence of Romans having a token-based sub-economy in any area, let alone brothels; there’s no real correlation between the numbers on one side of the coin and the acts depicted on the other; Romans didn’t have the same hangups about sex that we do, so the coins may not mean much of anything; and frankly, it’s so far back in time that we may never be able to definitively say what they were for.