Contrary to most answers here, the chances of a man getting infected from a single encounter with a woman are (relatively) low, at least compared to the near 100% risk most people here seem to imply:
"In general, the risk of a man getting HIV from a woman during vaginal intercourse in the United States is low--probably less than 1 out of 1,000 exposures will result in actual infection. This risk may be higher if the woman is having her period, and may also be higher in poor countries."
This is due to the fact that the virus can only be transfered via the entry of bodily fluids, and so the only way a man can get it is if an infected woman's vaginal fluid enters the man through his urethra, a cut, or a sore on his penis. It is much more likely for a woman or a gay man to get infected from vaginal or anal intercourse because the virus can be absorbed through the thin membrane of the vaginal and anal walls. It's a matter of surface area exposure.
I really hope nobody flames me for sharing this information. Of course I am not saying that men shouldn't always practice safe sex in a situation they aren't 100% sure about. After all, would you play Russian Roulette with a gun even if there was only a 1 in 1000 chance of it blowing your brains out? But exaggerating or misrepresenting the risks is not right either. Please practice safe sex, and get yourself tested, but more than likely you will be okay.