"If pregnancy is your concern, pulling out is not the most reliable or forgiving form of contraception. Re-inserting your penis after you've ejaculated (outside the vagina) is not safe either. Within the first few hours after ejaculation, there is usually some sperm left in the urethra. The amount left behind is nowhere near the amount of sperm in the typical ejaculate, but it still presents a possibility of pregnancy.
Spermicides, used alone, have the highest failure rates of all methods of birth control — even higher than withdrawal! If pregnancy prevention is important to you, spermicide alone is not the best contraceptive for you. However, you could use a spermicide in addition to withdrawal. The spermicide would offer a "back-up" in the event that you do not pull out in time. Spermicides come in many forms — foams, creams, jellies, suppositories, and vaginal contraceptive films. Some spermicides are made specifically for use with diaphragms, cervical caps, and IUDs, while other spermicides have been adapted for use alone. Most drug stores and pharmacies sell a variety of spermicides — you and your girlfriend may want to experiment with a few to find one that works best. Of course, as with any contraceptive, make sure you read the directions carefully, especially since spermicides have a time frame within which they are effective.
As a primary means of contraception, withdrawal has several disadvantages. First, there's the pre-ejaculate fluid (pre-cum) on which withdrawal will have no effect since it's released well before you ejaculate. This fluid may contain sperm and, thus, it does present a risk of pregnancy. Next, and perhaps more important, is the issue of consistency and self-control. Can you be perfectly consistent at withdrawing each time you have sex? If not, then what's done is done. Of course, you do have the option of emergency contraception (the Emergency Contraception Hotline is 1.888.NOT.2.LATE).
However, on the positive side, withdrawal does have its advantages. It's free and always available. Second, no side effects are associated with this form of contraception. Estimated failure rate for typical use (meaning you're not totally consistent) is around 20 percent. When practiced perfectly, some researchers estimate the failure rate to be around 5 percent. This means 5 to 20 women out of every 100 who use withdrawal as their contraceptive method get pregnant.
If you haven't already, perhaps you and your girlfriend could talk about your concerns and work toward reaching a mutual decision on what form(s) of contraception and/or safer sex you both want to use. Besides the issue of STIs, you need to assess how willing you are to risk having to deal with a pregnancy. Talk about what each of you thinks you would do if a pregnancy occurred. Withdrawal is definitely less effective than, say, the pill. But, if both of you are okay and comfortable with the risk involved, then you may decide that withdrawal is a good method for you. To make a decision like this, it is helpful to have as much information as possible. You could speak with a health care provider about your contraceptive options and choices. You will also find many questions related to birth control in Alice's Sexual Health archive."
Taken from Go Ask Alice, hope it helps.