condom slipped a bit

today when my bf and me had sex the condom slipped down a bit when he pulled out after he came. it was still on just not at the same place it usually is. I heard that you should take a plan b when the condom slipped. now my question is if I really need a plan b and why? the condom did not leave his penis it just slipped down a bit. I'm confused how sperm could get out this way?