How deep can the average anus be penetrated without injury? And how far stretched? Are there any guidlines, whether by measurement, specific feelings, or anything else?
As with anything, a good general rule is: If it hurts, STOP!
For more detailed information, run a search of threads on Lit (this topic is discussed in depth here and in the BDSM room) or a website search. There are several good sites with both practical and clinical information.
