Well, from what I can gather in your rather odd scenario (who on earth put semen in your beer?), I would say that chances of becoming infected are pretty much zero. This is why -
- you say that it was half an hour old. Hiv would not survive this long and in fact, only survives at most just a few minutes outside the body. You can rest assured that if there was virus in the semen, it was dead. When it's dead, there's no chance of infection.
- swallowing semen is not as dangerous as it might seem, since the stomach acids destroy the virus immediately. The risk is when the semen is fresh (as in just 'emerged') and can be absorbed through the mouth, in a small cut perhaps. This has happened. However, please read the first statement again. After half an hour, the hiv will be dead.
- the chances are excellent that the semen was not infected. But considering that this is not a dangerous scenario, this is irrelevant.
Don't worry.
P.S. I remember it happened in american pie, and did you see him worrying about anything else except how gross it was?