Myth # 1: "She can't get pregnant if I don't ejaculate inside her."
This relic of the "I Only Put It In a Little" School of Rhetorical Reasoning has accidentally conceived hordes of children just before intentionally soiling tons of bed linens. The high failure rate of this most messy of birth control methods is due to the lubricating presence of pre-ejaculate fluid, which leaks out of your penis before ejaculation and teems with more than enough sperm to impregnate a woman. So if you're relying on the "all-hands-on-deck" Pull 'n' Shoot method as contraception you should let your partner know you're a card-carrying member of Half-Assed (Un)Planned Parenthood by wearing a t-shirt proclaiming your true mindset: "Lord, I was born a gamblin' man."
Myth #2: "I can't get pregnant if we have sex standing up."
This is not what we mean by knocked up. Sir Isaac Newton wasn't much of a sexual research scientist, but it's safe to assume that the modern calculation of gravity -- still 9.8 meters per second, squared (and still holding, as of last Tuesday) -- is not enough to delay semen from rocketing toward your fallopian tubes. Having vertical sex won't increase your chances of dodging a hail of seminal bullets, either, so take every safe(r) sex precaution you would use for horizontal sex. Oh, and lift with your legs, not with your back.
Myth #3: "I can't get pregnant having sex in water."
Why not? The creatures of the seas do. While your attention might be focused on maintaining sufficient lubrication in an aquatic environment, your major concern should be potential pregnancy and disease from an "unprotected" partner; water wings do not count as safe sex paraphernalia, and just because you're in a body of water does not mean that any number of sexually transmitted diseases can't find a home inside your body.
Consider that a sperm's will-to-power is a simple yet potent one: swim or die. It's all about the relentless pursuit of fertilization, and millions of them can survive inside your body for almost three days as they swim upstream to deliver their chromosomal payload. Do you really feel safe gambling that some water -- spiked or not with pool chemicals -- and your reproductive tract can stave off a siege on Mount Ovum by as many as 300 million determined sperm?