If you don't want your girlfriend to get pregnant, you should definitely use a condom or other type of birth control. You could use spermicide or she can get on the pill, but really you should be having safer sex. Also, a girl can get pregnant on the pre-ejaculate, too, so whether you let the full load out inside of her or not, she could still get pregnant. I know it probably feels better without the condom, but I think that if there is less risk of pregnancy, then it's worth it to use it. Condoms also stop the spread of STIs, though I'm not saying that either of you have one.