Anal sphincter

Can regular anal sex or regular stretching the anal sphincter cause a loss of erection hardness.

I don’t think so.

Have a look at the book on anal health by Jack Morin. You can get it from Amazon or free through various torrent downloads. For some men stimulation of the prostate causes an erection to be lost but it is temporary

I’ve had this happen, though not from direct stimulation. My girlfriend running her nails up my leg, to my balls, makes everything quiver and contract up in there, and yeah, the erection dies. Does feel nice though…and an hour or so before sex can produce…messy…results.