I'm going to jack off while my girlfriend is asleep
That's horrible. Why would you do that when she's asleep?
Because she has such a crap sleeping posture and she rubs her face on the pillow and it's making me horny
It is normal to pleasure yourself but it is not okay to do it on the same bed when the other person is asleep. It is upsetting to know somebody did that over your body.
Never would have thought sleeping posture was so important.
If you need to do it, try to do it on a different room or somewhere private as doing it with someone else in the room can be considered as sexual harassment. If she suddenly wakes up, she may not understand what is going on and may report you to the police and get you arrested.