Condoms and the withdrawal method are, according to the newest study by Contraceptive magazine, are both found to be about 60% effective with regular usage, and 75% with perfect usage. Unfortunately, it is rare to have perfect timing with the withdrawal method. I'm sure that I don't have to point out how low those rates are when you consider that most people have sex about 190 times a year.
Just in case, I will. That means that if you have sex 190 times a year, about 42 times you're method of birth control is probable to fail.
Combined with birth control or the rhythm method, though, that changes.
Ultimately, though, if you're wanting a safe way to have sex without a condom (which is only safe if you have a verifiable copy of his STD check, really) you need to get on the pill or something.