Like I said, the chances are slim, but are possible and it does happen. Sure it's safer with a condom, but my point is it's not a 100% gaurentee. Sure 4 of 1000 seems like a small number, but when you think of how many condoms a couple could use in one night, 1, 2, 3, 4 , it adds up quickly.
Don't get me wrong, i'm not saying you shouldn't use condoms, you absolutley should!, i'm just saying that you shouldn't consider it a 100% gaurentee, and then say, well I used a condom, I can't have hiv, be pregnant, etc...