I am sure you have chanced upon the same news stories as I have: some fine, upstanding, and 180% heterosexual man is rushed to the emergency room with some large, foreign and strange object inserted irretrievably into his rectum. After having the object removed, the victim maintains with a straight face that they were trying to get the evaporated milk off of the top shelf of the pantry, and just happened to slip and fall in such a way that a potato ended up in their bottom. Obviously, this is not a very likely story. But it is, so to speak, the only game in town. Up until now. Not that I am judging anyone, but if you wish to stick things up your bottom and then deny that it was for some type of thrill, there are better ways to do it.
If I was ever caught in such a situation (which I am not planning to be), I would have a better story at hand. I would merely claim that I had an embarrassing and maddening medical problem that was driving me so mad that I was driven to soliciting medical advice from the internet, and found that some piece of folk wisdom suggested this as an unorthodox yet effective cure. If you all can forbid a bit of seriousness on this topic, as an eczema sufferer, I have been driven to scrub my hands until they are bloody with steel wool because of the itch. If I was confronted with someone who claimed that the internet had told them that a potato or an apple had starch or pectin or something that could sooth a sanity-ending itching in their posterior, I would probably nod sympathetically. And if a man said that he had read that the pressure of a shampoo bottle could compress one's prostate down to the size where urination was not a constant annoyance, many men would understand how someone could be driven to such distraction. The only danger here is that half the male doctors and/or paramedics don't run home to try it themselves and/or institute a double blind study of the effects of inserted shampoo bottles on prostate size. In any case, by admitting with some embarrassment to a minor personal problem, such as uncontrollable rectal itching or prostate problems, the larger embarrassments are thereby plausibly avoided.
Having concluded this, I would suggest everyone print this out, and then clip it out and carry it around in their wallets for future reference. Also, I have to say that no matter what your personal or sexual preferences, don't insert large objects in your rectum. Seriously, don't.