Last I heard they were only 80% effective for pregnancy and a sperm is a lot larger than the HIV virus. Imagaine a bottle with 10 asprin in it. Now imagine that someone tells you two of those "asprin" are really deadly cyanide poison but look exactly like the 8 asprin they nestle among. If you had a headache would you feel safe taking one of the "asprin"?
Or, imagine a 10-cylinder revolver. Would you feel safer playing Russian Roulette with IT than a standard 6-shooter?