Speaking from much experience, I would have to say that I haven't found too many guys (read that as none) that would like or have liked (even if by accident) my biting down on their penises while in the act of sucking on them. It can happen, but I would have to say that I have never been asked to bit down.
Coming from a guy who enjoys having his penis bitten, it is extremely arousing. I don't understand why people can bite a nipple and see how that could feal good for some women, but can't see that having a penis bitten would feal good for some men. it is the same type of arousal feal good of having your nipple bitten, or neck or any other area that might feal good for some people. it is just way more intense.
It's an uncommon kink related to masochism. It is dangerous but can be done fairly safely. The penis is tougher than it looks. It can handle a lot of abuse without immediate injury. Your teeth however can sheer the skin off someone's arm with minimal effort and can easily tear a penis from the torso if enough force is applied. Many men into CBT experience ED at a younger age. Theoretically the repetitive intense pain damages their nervous tissue.
Bite but don't grind or pull. Pressure is much less dangerous than torsion, less likely to damage the ligaments of the penis, less likely to break the skin cause actual injury.
It is possible that he has a sexual need for pain but just no experience with other avenues of extreme sensation. See if he'll experiment with safer forms of pain or things that wig you out less. The buttocks and back can take some surreal punishment without serious injury, it's the same endorphin high. Clothes pins can cause much more consistent and measurable pain than your teeth with a safter wider surface area. And of course never take things outside of your comfort zone just to please him.
Well I wouldn't like it at all if she bit it hard and caused pain.
What is REALLY sexy is if she puts it in her mouth from the side and bears her teeth and just barely put pressure down with the teeth while looking you in the eyes and kinda'winking like she is going to do it. But actually biting it I wouldn't like.