In response to the enquiry: “Why do I tense up having anal sex?”
Butt muscles work both ways.
In difficult defecation when the fecal matter is very dry and hard and clumpy, one tenses up the body muscles to try to force the fecal matter out.
Anal penetration and the anticipation of anal penetration cause the butt muscle to tighten.
Tightening of the butt muscles means that the muscle groups of the torso or upper body, including the neck muscles, tense up and tighten.