*cough*
pre-cum is not more potent than "regular sperm". It's all the same shit.
Pre-cum just can contain a few little spermies that happily swim their way up there, figure it as a leak of sorts. Otherwise the pull-out method (ok, when done correctly. *cough*) would not be as effective as it is (somewhere around 80%, IIRC).
Sperm in most city or pool water will not fare too well either. The chlorene does a number lifeon their expectency, and is pretty damn effective at killing them. I forget the odds of it actually happening in pure water, but it's something insane like 1 in a few hundred million. Does make you wonder though.
Hell, most teenagers are so damn fertile birth control is not nearly as effective as it is on older women. It's a well established fact now that birth control forms are a couple percentage points lower in effectiveness on teens, than in the normal population. Add to that "oops I missed a day with the pill" sort of stuff teens are more apt to do, and damn.
Believe me, I studied this shit a LOT. And not I have a kid.. Go figure.
-Phil