Back in high school your sex ed teacher told you that pulling out doesn't work to prevent pregnancy. He or she lied to you.
Whenever birth control topics pop up on website forums there are always a bazillion people who are like "My friends use the pull out method! Why are they so risky! How can I teach them the One True Path of hormones and/or condoms?"
I can see why they feel that way. Teachers make withdrawal seem terrifying for one reason - they don't trust 17 year old boys to have the self control to actually do it.
The truth is that for adults it is about as effective (and sometimes more so) than methods that people have no problem using.
Consider these facts:
In perfect use condoms have a 2 percent failure rate. Pulling out in perfect use has a 4 percent failure rate.
In typical use condoms have a 15 percent failure rate. In typical use pulling out has an 18 percent failure rate.
In perfect use a diaphragm with spermicide has a 6 percent failure rate - which is higher than pulling out.
I know this seems odd and the question that always comes up is "doesn't pre-ejaculate have sperm in it?"
The answer is "not usually." Pre-ejaculate only has sperm in it if the dude hasn't urinated since his last orgasm. Once a guy shoots some sperm ends up left in his tubes. Taking a pee completely flushes all of that out.
Pre-ejaculate itself contains no sperm and comes from a completely different part of the body than sperm does.
The bottom line - unless you are going for disease prevention - your friends who are using the pull out method are not being all that much riskier than anyone who is not using a hormonal method.
You can read more about the effectiveness of different methods here.