Biting (Even if it is someone's idea of a joke)

My friend was over at my house today...he has a bit of an odd habit. When he has to go I give him a hug, then because he thinks that I THINK am a vampire (I have yet to come out and say that it is the truth) he thought he would be funny to lightly bite my neck (on the right side, not where the carotid artery is) while we were still embracing...The first time this happened I was like, "Ok, now it's MY turn" and I lightly bit him back (on the left side, the one that the carotid artery IS). I think there is something in our natures (a vampire's that is) that makes us "overreact" when we get bitten....something to do with the fact that we feel that someone is trying to take our blood...but we DO tend to overreact... I also think that for us there is something sexual about it....probably because basically as soon as we become old enough to be interested in sex we make the connection between blood and sex....sort of like good food and sex for humans....I was lucky that I caught myself. I still bit him (it would have looked odd if I did not given what I had said...) but I forced myself to do it in an extremely light manner (I probably bit him with less force than he bit me). The problem with all of this is that the friend does not know how close I was to biting him (rules be damned) and taking his blood. Every time he does it it gets harder and harder to control myself when I do it back (he expects me to do it...and at this point I probably could not stop myself even if I wanted to). I do not wasnt to hurt this person because he means a great deal to me, but I cannot bring myself to tell him to stop...even if it means my biting him and seriously injuring him.