There’s nothing like being sexually misinformed.

It can foil your sexual response. It can leave you pregnant or fielding an infection. And it can make you feel pretty silly, especially when you realize you have been wrong all along.

So to make sure you don’t go another day in the dark, here’s the real deal on some of the most important sex facts:

1. Myth: Viagra is 100 Percent Effective

When it comes to taking Viagra, or other sildenafil medications, which treat erectile dysfunction, men tend to think that it’s 100 percent effective. All you need to do is pop a pill, sit back and enjoy, right?

Fact: A desire component is needed for males to become sexually aroused and attain erection. Unless that's there, you're headed for disappointment.

The consequence: Half of the men who try using these drugs end up discontinuing them by the end of the year. This is partly due to inadequate communication with their partner and inadequate education on what to expect.

2. Myth: A Virgin's Hymen Always Breaks

Many people think that a female’s hymen is broken the first time she has intercourse.

Fact: This is not necessarily so for every gal. Depending on the female, this thin skin that stretches across the vaginal opening may be anywhere from nearly nonexistent to covering everything. It is not always torn during intercourse. Actually, 19 percent of sexually active females have no visible tearing.

This is important to know, since many cultures think the presence of a hymen is an indicator that a female is a virgin. Girls are born with hymens of various sizes and openings. Some may appear to have no hymen at all. Others have their hymen stretched from activities like bicycling or horseback riding.

3. Myth: Withdrawal = Good Birth Control

Couples have relied on the withdrawal method as a form of birth control for centuries.

Fact: Pregnancy can occur any time unprotected sex is had, whether or not a male has climaxed. Withdrawal is therefore not recommended as a form of birth control, especially for males who are sexually inexperienced.

The consequence: About half of the 6.4 million pregnancies that occurred in the United States in 2001 (the most recent year for which good data was available) were unplanned. Definitely something to think about before relying on this method.

4. Myth: Oral Sex is Safe Sex

Plenty of people engage in oral sex because they think it does not put them at risk for sexually transmitted diseases.

Fact: Unprotected oral sex puts both partners — whether giver or receiver — at risk for a number of STDs.

The consequence: While the risk of transmission from oral sex is generally lower than unprotected intercourse, lovers still have to worry about STDs like herpes and HIV.

5. Myth: You Can't Get Pregnant if You Aren't Ovulating

She’s not ovulating so she can’t get pregnant — Not!

Fact: While pregnancy is likeliest to occur during the six days leading up to, and including, ovulation, a female can get pregnant at any point in her menstrual cycle. This includes the week of her period.

Even if couples want to take a chance and avoid unprotected sex around day 14 of her menstrual cycle (when she is most fertile), the fact that many women have irregular cycles makes this a dicey decision. Even women who have regular menstrual cycles may not ovulate on the same day each month.

6. Myth: The Pill Protects Against STDs

Many females, especially young women, believe using a contraceptive pill will protect them not only from pregnancy, but also from sexually transmitted diseases.

Fact: All hormonal birth control methods, including the pill, provide protection only against pregnancy. They do not protect either lover from the transmission of infections. A male or female condom is the only way to protect against STDs when sexually active.

Dr. Yvonne K. Fulbright is a sex educator, relationship expert, columnist and founder of Sexuality Source Inc. She is the author of several books including, "Touch Me There! A Hands-On Guide to Your Orgasmic Hot Spots."

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