Let's see if we can help the liberal out on this one (and anyone with a probabilities background is welcome to join in).
According to planed parent hood, a condom is 90% effective at preventing pregnancy.
A woman can only become pregnant approximately 6 days out of every month.
That means a condom only has to work 20% of the time, and during that time, it is only successful 90% of the time.
AIDS is transmissible 100% of the time. My statistics and probabilities haven't been used since college, but I put a condom at about 18% successful at preventing AIDS.
Hardly a ringing endorsement, but it hasn't stopped the government from spending millions to promote “safe” sex!