The anal sphincter is actually TWO sphincters, one stacked on top of the other. The outer sphincter, the “human sphincter,” is under conscious control. You can wink it open and closed right now if you want to.
The inner sphincter, though, is the monkey sphincter. It’s not under conscious control. It reacts to the environment by winking tight in response to novelty, stress, and change, and risk.
And it takes the outer, human sphincter with it.
Because it’s a monkey, you can’t talk it out of feeling untrusting, stressed, or surprised, you can’t reason with it. So a key to pain-free pleasure-full anal penetration is keeping the monkey sphincter calm and happy. The person being penetrated should always feel trusting and in control. They should be the person controlling how fast and deep penetration happens, and the pentrator should just stay still and calm.
Always go slowly and gently with the anus, and when it winks in response to novelty, stress, change, or risk, stay still and calm and allow the wink to pass. Work WITH the wink. Allow it.