Hello.
If you're trying to get pregnant, the pull-out method is a very effective way. In fact, we sexologists have a special word for people who use that method. We call them "parents."
The reason that pulling out is ineffective is that there can be millions of sperm in a man's pre-come, left over from previous ejaculations. Not only that, but it's often difficult for men to judge just when they're about to ejaculate. After all, most of the time during sex, the little head takes over the thinking from the big head, and all judgment goes out the window.
Just because you haven't become pregnant yet doesn't mean you won't one of these days. So if you want to avoid pregnancy, I suggest you learn more about effective forms of birth control and risk reduction. Check out these pages on this site for more information:
risk reduction:
http://www.medhelp.org/medical-information/show/1723/Safe-sex-
condoms:
http://www.medhelp.org/medical-information/show/2255/Condoms
birth control and contraception
http://www.medhelp.org/forums/Birth-Control-Contraception-/show/382