Yes, it can. If you mean that you keep your underwear on and go through the motions, it is NOT a foolproof barrier method to prevent sperm from entering your vagina.

As far as barriers for preventing pregnancy, male condoms are the most popular. Female condoms are also available (they look like a tube with circular wires at both ends and one end is closed off) but they can be very expensive. If you go to a gynocologist, you can ask to get fitted for a diaphram or cervical cap, but be warned that some spermicidal lubricants can cause problems. Please use condoms to prevent unwanted pregnancies at the very least and do not reuse them - you will need a new one every time!

Might I suggest sex education...

In order to get pregnant a good amount of sperm needs to get to your cervix deep inside your vagina. Once sperm gets exposed to air, it starts to die. Add to that the fact that the semen needs to get through the clothes, into your vulva, find your vaginal opening, move up to the cervix and then into the uterus to the ovaries. In your clothes there are chemicals left from washing it that will further kill the sperm.

You also have only a week during your cycle when sperm can make you pregnant so all this will have to be perfectly timed to get you pregnant.

It is very very very unlikely that you will get pregnant from "dry" humping but it is not impossible. It is better to use a condom or for him to keep his pants on and his penis inside his pants.