Sphincter atrophy

The changes were made to reality and it was completely normal and expected that this change would inevitably change the female anatomy.

Just like in stretched earlobes, the girls would start with a modest sized plug on their 18th birthday but as time went on, their sphincters atrophied and they needed larger and lager plugs to accomodate their loosing rear ends.

While a 30-something year old woman would still be pretty tight, as soon as they reached 45 and older most could comfortbly accomodate a fist in their asses without much discomfort.

Therefore as they reached their 60th birthday and technically weren't required to wear their plugs anymore, most woman kept wearing them to the grave. This was much more comfortable and less humiliating than wearing a diper or going though extensive kegel excercises to make them tighten back up again.

As a side effect, anal sex was less and less appealing for men with older partners because, while being more willing and open to anal fisting, the men simply couldn't feel the tighness of their wifes asses even when squeezing their sphincters.