"Precum" can cause pregnancy, but he has to be inside you for you to become pregnant. There can be some sperm in the "precum". There is no way sperm could find their way when he isn't even at the opening of your vagina.
When you are a virgin, it means that there is a piece of skin inside you that is still intact. When this skin is broken you are no longer a virgin. That is why there is usually some bleeding the first time you have sex, because the virginal barrier breaks. I would think that this barrier would also decrease your chances of becoming pregnant when he is only at the opening.
You cannot get pregnant if he still has his underwear on. If you want to prevent pregnancy, there are many method for doing so. Although "playing around" like you have been doing is not going to get you pregnant, passions can get out of hand and it's very close to getting to a danger level as far as getting pregnant.
Of course the best and 100 percent sure method to not getting pregnant is not having sex.