My girl gets me with her teeth pretty much every time. It's not my favorite feeling in the world but it's a damn sight better than not getting head at all. As long as you're not getting all cheese grater on him I wouldn't worry about it.

Also you could try licking him like a paddle pop when your mouth needs a rest. Or stick out your tongue and kind of whack his dick down on it. You'll send him to the moon. **** I wish someone would give my girlfriend this advice.