I would not way this guy is a nut job at all. A lot of guys like it a little rough. I actually find some biting on the pens head a major turn on form of play. Not hard of course to where lasting marks are made, but the head of a penis is reasonably tough. Now the twins below that a different matter. Any biting there needs to be VERY light. This should not be a big issue. Just discuss it. I have a partner who likes it gentle, knows I like it more rough, and she has even had me tie her up. It is a matter of mutual trust and respecting what each partner likes. I doubt biting would lead to orgasm though no one has tried to that level. It is more a form of for play just like biting anywhere else is. Just not hard enough to leave a mark. So bit away.
