Here is an excellent summary, giving many references.
Note that the conclusions are in line with what Wander and I are saying.
It is possible to find many articles that say words like "If you have unprotected sex, sooner or later you will get HIV". However, although those statements are not technically false, they do little to help one understand one's risk.
Here's another way of looking at it:
The chance of getting run over by a car is very low, but every person who dies from it gets run over in exactly one incident. For some of them, it will be the very first time they crossed the road. For most, it will be just one road-crossing out of thousands they have done. And of course, nearly 100% of the people who cross the road do not get run over by a car throughout their whole lives.
There is only one way to guarantee not getting run over by a car: never cross the road, or, cross the road while wearing a tank, so that if you are run over by a car, you will not be hurt by it.
There is a cost to your lifestyle if you decide never to cross the road. But it does pretty much guarantee that you won't get run over by a car.
So, for unprotected sex, the studies I referenced above say: Some will catch HIV in their first unprotected encounter. For others, it will be just one encounter out of many they've had. And the vast majority will never catch it, despite having unprotected sex their whole lives. This is because the transmission of HIV from female to male during unprotected vaginal sex is very low, and for oral sex, even lower. For anal sex, somewhat higher.
Final thought: Thank goodness that HIV doesn't transmit very easily. But many other lesser STDs transmit much more easily, so it isn't just HIV one has to worry about. At the end of the day, we all choose to cross the road, but like me, many punters decide that the risk of HIV or some other infection are still too high (or at least we don't quite have the confidence that those risks are low enough) for us to engage in unprotected vaginal or anal sex. But the majority of us seem comfortable participating in unprotected oral sex, even though the risk is likely not absolutely zero.
Sorry if this is frustrating because it doesn't supply a clear yes/no answer. But that's life.
Gherk