I know I've gotten a number of marks on some pipe's stems I have. I think that not all stem material is created equal, so some is harder than others and more resistant to bite marks, and some of us are biters/clenchers more than others as well.
If you're a hard clencher try some softy bits. I fine them more comfortable than clenching a bare stem especially a lucite stem, and never smoke without them.
I am a major clincher. As I moved up the quality and cost levels of pipes I decided to get some softies to protect the investment so to speak. It took some serious adjustment time getting used to the new mouth feel but now I have to have them on everything.