You can create something very close to a long futures position, with a delta of 1, no time value, and a margin requirement, by creating a synthetic long -- buy a call and sell a put at the same strike price. If you do this on a stock where the options have a narrow bid/ask spread you will find that the gain/loss tracks very closely with the stock. Any time decay, and any increase or decrease in the volatility will affect both options equally, so cancels out. This is not due to some theoretical pricing model, any significant deviation from put-call parity creates an opportunity for risk free profits by arbs.