Penis Vibrator
My high school girlfriend, my ex-girlfriend from college, and my present girlfriend all do the same trick in bed...
They take my penis and use it on themselves to get off as though it was a vibrator. Each one of these girls initiated this action on their own without my prompting. Each one of these girls is totally unlike the other in terms of everything. Is this a normal thing all girls do?
They are stimulating their clitoris by doing this, and it gets them wetter. Take your time with stoking the fire, and you will probably see they stop doing it.
If you like this, and you are all having safe sex (yes, you should wear a condom even if there is no penetration!), then go for it!
However, you almost make it sound like they are not having sex with you, but are just borrowing your penis. You, as a person, should be involved (unless, again, you really enjoy this).
You might want to try moving around a bit more. If you actually move toward her with your penis and do something to her with it, she may enjoy that.
Again, wear a condom even without penetration!
Make an initiative and stick it to your girl (haha) so you won't end up being the vibrator. On the other hand, if you're just plain uninterested then perhaps you should say something.