They never worried when John brought his toothbrush over to Jack's. The fact that it might change something never crossed their minds. The brushing of teeth was necessary but the sharing of toothbrushes was completely Out Of The Question. It would confuse the poor toothbrush out of its wits and Jack would probably have to get a new one, which is a complete pain (the process one has to go through just to get it adjusted to the mouth is insane). The only issue they really had was figuring out who would take whose toothbrush to whose house. Jack, of course, automatically pointed the finger at John. Jack, after all, usually did the "taking home." And John, of course, flat out refused. Jack bottomed (...most of the time), after all.
Later, of course, John surrendered, a mistake which would later be used against him in the "The Great Wife Debate." Anyway, as feminising as it apparently was to leave one's toothbrush at his whatever's house, that is as far as either of them thought it would go.
The real issue, of course, is that neither of them thought this through: the trouble of there not being a woman to be the voice of reason (although Jack could and would dispute that). You see, they didn't really think about what John would use once he spent the night at is own house: his toothbrush would not be there. It would be at Jack's. Thus, the underlying problem- and reason for this tale- begins.
Note: Woah, that's really short. But that's okay; it's allowed to be. (Prologues get special privilages.) Anyway, I'll update again soon. Hopefully it won't be as short this time. Please review and critique; I'm always trying to improve as a writer.