The gerbil, now trapped inside your anal cavity, thrashes around, desperate for air. It is this thrashing that provides pleasurable sensations. Once the gerbil is dead, remove it by pulling on the string. Some straight people have a peculiar need to believe certain sex acts, usually disgusting ones, are practiced only by gay men, despite evidence to the contrary. Fisting, for instance.
Straight people can and do fist.
Child rape, for instance. Inserting a wet cardboard paper towel roll into your ass is simply not possible, as anyone who's ever put anything in their ass can tell you. Now, I feel I can write with some authority that no one has ever actually stuffed a gerbil up their butt, perhaps with more authority than I can write that God and angels do not exist. Both in my professional and personal life, thousands of guys have freely admitted to doing the most out-there, dangerous, risky, stupid, kinky stuff. But not once in all these years has anyone ever told me that he, or anyone he knows, or anyone he knows knows, has ever put a gerbil in his ass.