Condoms break, and there's also the potential for a heat-of-the-moment decision to have sex anyway if a condom isn't handy, breaks when putting it on, etc. You also need to know with confidence that your partner(s) are not HIV positive
. ASK! And if he refuses to answer, or says he is positive, it's sayonara, buddy. (If he is negative, still use condoms, unless and until he and you commit to an exclusive relationship.