Originally published April 27, 2011.

I'm a 24-year-old straight guy. I've been with my girl for three years, and things are great — great sex life, great communication, etc. We have lots of sex — but for the last year or so, she has not been on birth control and we have not been using condoms. We're not against the idea of a child, but we aren't currently going for it. I was always told that pulling out was a 100 percent ineffective method of birth control. So my question is, I guess, could there be something wrong with one of us? How could we have unprotected sex for a year without getting her pregnant? We both really want children eventually and are worried it might not happen.

Sent From My iPhone

Withdrawal is a much more effective birth control method than most sex advisers are comfortable acknowledging. But facts are facts: A comprehensive study conducted by researchers at the Guttmacher Institute found that withdrawal was almost as effective a birth control option as condoms. ("Better Than Nothing or Savvy Risk-Reduction Practice? The Importance of Withdrawal," Contraception, June 2009.)

"If the male partner withdraws before ejaculation every time a couple has vaginal intercourse, about 4% of couples will become pregnant over the course of a year," the authors of the study wrote. That compares pretty favorably with the 2 percent of straight couples who will become pregnant using condoms perfectly over the course of a year.

In the real world, of course, very few people do anything perfectly. When you take mistakes, leaks, and broken condoms into account, researchers estimate that 17 percent of straight couples who rely on condoms will become pregnant in any given year. Not all withdrawers use withdrawal perfectly, either — amazingly enough, some guys get distracted and forget to pull out as their orgasms approach — but the research shows that just 18 percent of straight couples who use withdrawal will get pregnant in any given year.

So odds are good that you're not infertile, SFMi, just lucky.