First, let's be clear about what you're trying to prevent here. If you're looking to prevent sexually transmitted infections (STI), then stick to a condom (and keep it on the whole time), because pulling out is NOT going to provide protection against infection. If you're just looking to prevent pregnancy, then you'll be happy to know that withdrawal method (pulling out) is a potentially viable alternative if other contraceptives don't seem to be ideal for you.
There are certainly some advantages to this method: You don't need a prescription or any sort of device, it's free and always available, and there are no side effects. But does it work? To figure out how well a contraceptive method works, you should consider typical-use rates and perfect-use rates. Perfect use is if you use the method exactly as it is intended. (People are rarely perfect; therefore this rate is rarely accurate.) Typical-use rates are if you use the method the way the average person would typically use it. With typical use, about 27 percent of women who use the withdrawal method experience an unintended pregnancy within the first year of use (4 percent with perfect use).
The difficulty with pulling out is that, to do it perfectly, the guy must be really in tune with when he is going to ejaculate.
Although most men would like to think they've got a lot of control with their ejaculation timing, the majority of them don't. So, how can you tell if he has perfect control of ejaculation? Well, you can't. Testing this method out while using some other birth control method might be wise. You could use spermicide in addition to pulling out just in case he doesn't pull out in time (then it will hopefully kill any of the sperm that make it in). However, spermicide on its own is actually a pretty poor method, so make sure you do combine them.
Also, some people worry about pre-ejaculatory fluid (or "pre-cum"). This isn't much of a problem unless you have sex a couple of times close together. Once he ejaculates, there is usually some sperm left in the urethra, so if you've had sex a couple of times in a row (without having him urinate in between), then you risk having some active semen in the pre-cum. Otherwise, that pre-cum shouldn't contain active semen.
This method requires putting a lot of trust in your partner, so it really is a mutual decision that should be discussed and decided upon together. Pulling out is certainly less effective at preventing pregnancy than, say, the pill or condoms. But, if you are aware of the risk you're taking and the pros outweigh the cons of this method, it might be right for you!
