Spermicide.. fairly self-explanatory. It kills the little spermatozoa before they go into your SO's uterus and fertilize her egg. Condoms often have a spermicidal lube preparation on them (that's the point after all). I'm sure that the Roman Catholic church have something to say about such things, as they are opposed to contraception... remember, there's millions of sperm in every shot, so you are not a beautiful and unique snowflake.