This reminded me of something from the playboy advisor (my boyfriend has a subscription and I have no shame in saying I love to read it for the articles... Seriously!) anyway, a guy asked:
"i have been using the withdrawal method to keep from getting my girlfriend pregnant. That is, I come on her stomach. She takes a shower after sex, and i'm wondering if the water can carry the sperm into her vagina. Have you ever heard of that happening?"
.Playboy responded:
"no. Even with the best of intentions, it's much more likely you'll get her pregnant before she hits the shower. Coitus interruptus is better than nothing, but that's about the only good thing we can say about it. The technique's failure rate is estimated to be about four percent with perfect use, meaning that over a year's time, four of 100 women using it will get pregnant. However, given that highly aroused men are involved, a 19 percent failure rate is thought to be more realistic. About the only birth-control methods more risky are the rhythm method and crossing your fingers. For many years researchers thought the high failure rate could be attributed to pre-come, which is produced by the .Cowper's gland and emerges during foreplay in quantities ranging from a few drops to a teaspoon, probably to lubricate the urethra and head of the penis. However, several small studies have found no sperm or only immobile sperm in the fluid, so human error remains the chief culprit."