Buck Wolf informs us that the old chestnut about Mountain Dew preventing pregnancy is still very much with us in the enlightened 21st century. Some teens surveyed in Florida admitted to believing not only that Mountain Dew is an effective contraceptive, but that swallowing a capful of bleach will prevent AIDS, as well. Some pundits blame the kids' ignorance on Florida's abstinence-only sex education program, but in fact these myths have been widespread for years.
Read more about it:
• Is Our Children Learning Sex Ed? - Weird News
• Mountain Dew as Contraceptive - Urban Legends
Read more about it:
• Is Our Children Learning Sex Ed? - Weird News
• Mountain Dew as Contraceptive - Urban Legends

Comments
I was surprised to see no mention of what first occurred to me, and may have been the source of all this misinformation. Is it not possible that it is the manner of application that is the source of this legend; not that the sodawater is to be drunk, but that it is applied directly within the woman’s vagina after intercourse as a spermicide? I know the phosphoric acidity of Coke, Pepsi, and similar drinks can be used as a make-shirt douche, but I’m uncertain as to the effectiveness of other drinks such as Mountain Dew. Vinegar (which is acidic) is another traditional douche. Please expand on this idea as a source of the misunderstanding.
While I would love to have a reference, there is a good chance it’s merely word of mouth or a rumor that is self-perpetuating.
While the acidity idea may have some validity, why acknowledge it as a method when it’s not as certain (or safe) as a condom. I’m sure crocodile dung and honey was somewhat effective for ancient Egypt (if I remember that correctly), but I wouldn’t encourage it now.