The risk of getting HIV if you're a guy having normal intercourse with a woman is something like 1/200 - 1/300.
Meaning: out of 200 to 300 cases, 1 would result in transmission. Factors are duration of intercourse, thrusting speed/force, whether or not the woman has other STDs (because this increases virus concentration in vaginal fluids, perhaps?), and probably a bunch of other things.
But the bottom line is, that for a man to contract HIV through one single act of sex with a woman is very unlikely.
Wikipedia has even lower numbers than the ones I remember. Wikipedia list 5 cases of out 10 000 exposures to an HIV source, woman positive, man exposed, vaginal sex.
"impossible" is the wrong word to use! That's the whole point, and the potency of HIV. The risk of so low, but sex is so common, that it is bound to spread eventually. As long as there is a slow but steady increase in numbers of exposures, the virus is "winning". So while the risk of spreading HIV from a woman to a man is only 1/300, or even lower, a woman'll have more than 300 acts of intercourse...virus spread.
